fix(copilot): exchange raw GitHub token for Copilot API JWT

Raw GitHub tokens (gho_/github_pat_/ghu_) are now exchanged for
short-lived Copilot API tokens via /copilot_internal/v2/token before
being used as Bearer credentials. This is required to access
internal-only models (e.g. claude-opus-4.6-1m with 1M context).

Implementation:
- exchange_copilot_token(): calls the token exchange endpoint with
  in-process caching (dict keyed by SHA-256 fingerprint), refreshed
  2 minutes before expiry. No disk persistence — gateway is long-running
  so in-memory cache is sufficient.
- get_copilot_api_token(): convenience wrapper with graceful fallback —
  returns exchanged token on success, raw token on failure.
- Both callers (hermes_cli/auth.py and agent/credential_pool.py) now
  pipe the raw token through get_copilot_api_token() before use.

12 new tests covering exchange, caching, expiry, error handling,
fingerprinting, and caller integration. All 185 existing copilot/auth
tests pass.

Part 2 of #7731.

tests/hermes_cli/test_copilot_token_exchange.py

@@ -0,0 +1,159 @@
+"""Tests for Copilot token exchange (raw GitHub token → Copilot API token)."""
+
+from __future__ import annotations
+
+import json
+import time
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def _clear_jwt_cache():
+    """Reset the module-level JWT cache before each test."""
+    import hermes_cli.copilot_auth as mod
+    mod._jwt_cache.clear()
+    yield
+    mod._jwt_cache.clear()
+
+
+class TestExchangeCopilotToken:
+    """Tests for exchange_copilot_token()."""
+
+    def _mock_urlopen(self, token="tid=abc;exp=123;sku=copilot_individual", expires_at=None):
+        """Create a mock urlopen context manager returning a token response."""
+        if expires_at is None:
+            expires_at = time.time() + 1800
+        resp_data = json.dumps({"token": token, "expires_at": expires_at}).encode()
+        mock_resp = MagicMock()
+        mock_resp.read.return_value = resp_data
+        mock_resp.__enter__ = MagicMock(return_value=mock_resp)
+        mock_resp.__exit__ = MagicMock(return_value=False)
+        return mock_resp
+
+    @patch("urllib.request.urlopen")
+    def test_exchanges_token_successfully(self, mock_urlopen):
+        from hermes_cli.copilot_auth import exchange_copilot_token
+
+        mock_urlopen.return_value = self._mock_urlopen(token="tid=abc;exp=999")
+        api_token, expires_at = exchange_copilot_token("gho_test123")
+
+        assert api_token == "tid=abc;exp=999"
+        assert isinstance(expires_at, float)
+
+        # Verify request was made with correct headers
+        call_args = mock_urlopen.call_args
+        req = call_args[0][0]
+        assert req.get_header("Authorization") == "token gho_test123"
+        assert "GitHubCopilotChat" in req.get_header("User-agent")
+
+    @patch("urllib.request.urlopen")
+    def test_caches_result(self, mock_urlopen):
+        from hermes_cli.copilot_auth import exchange_copilot_token
+
+        future = time.time() + 1800
+        mock_urlopen.return_value = self._mock_urlopen(expires_at=future)
+
+        exchange_copilot_token("gho_test123")
+        exchange_copilot_token("gho_test123")
+
+        assert mock_urlopen.call_count == 1
+
+    @patch("urllib.request.urlopen")
+    def test_refreshes_expired_cache(self, mock_urlopen):
+        from hermes_cli.copilot_auth import exchange_copilot_token, _jwt_cache, _token_fingerprint
+
+        # Seed cache with expired entry
+        fp = _token_fingerprint("gho_test123")
+        _jwt_cache[fp] = ("old_token", time.time() - 10)
+
+        mock_urlopen.return_value = self._mock_urlopen(
+            token="new_token", expires_at=time.time() + 1800
+        )
+        api_token, _ = exchange_copilot_token("gho_test123")
+
+        assert api_token == "new_token"
+        assert mock_urlopen.call_count == 1
+
+    @patch("urllib.request.urlopen")
+    def test_raises_on_empty_token(self, mock_urlopen):
+        from hermes_cli.copilot_auth import exchange_copilot_token
+
+        resp_data = json.dumps({"token": "", "expires_at": 0}).encode()
+        mock_resp = MagicMock()
+        mock_resp.read.return_value = resp_data
+        mock_resp.__enter__ = MagicMock(return_value=mock_resp)
+        mock_resp.__exit__ = MagicMock(return_value=False)
+        mock_urlopen.return_value = mock_resp
+
+        with pytest.raises(ValueError, match="empty token"):
+            exchange_copilot_token("gho_test123")
+
+    @patch("urllib.request.urlopen", side_effect=Exception("network error"))
+    def test_raises_on_network_error(self, mock_urlopen):
+        from hermes_cli.copilot_auth import exchange_copilot_token
+
+        with pytest.raises(ValueError, match="network error"):
+            exchange_copilot_token("gho_test123")
+
+
+class TestGetCopilotApiToken:
+    """Tests for get_copilot_api_token() — the fallback wrapper."""
+
+    @patch("hermes_cli.copilot_auth.exchange_copilot_token")
+    def test_returns_exchanged_token(self, mock_exchange):
+        from hermes_cli.copilot_auth import get_copilot_api_token
+
+        mock_exchange.return_value = ("exchanged_jwt", time.time() + 1800)
+        assert get_copilot_api_token("gho_raw") == "exchanged_jwt"
+
+    @patch("hermes_cli.copilot_auth.exchange_copilot_token", side_effect=ValueError("fail"))
+    def test_falls_back_to_raw_token(self, mock_exchange):
+        from hermes_cli.copilot_auth import get_copilot_api_token
+
+        assert get_copilot_api_token("gho_raw") == "gho_raw"
+
+    def test_empty_token_passthrough(self):
+        from hermes_cli.copilot_auth import get_copilot_api_token
+
+        assert get_copilot_api_token("") == ""
+
+
+class TestTokenFingerprint:
+    """Tests for _token_fingerprint()."""
+
+    def test_consistent(self):
+        from hermes_cli.copilot_auth import _token_fingerprint
+
+        fp1 = _token_fingerprint("gho_abc123")
+        fp2 = _token_fingerprint("gho_abc123")
+        assert fp1 == fp2
+
+    def test_different_tokens_different_fingerprints(self):
+        from hermes_cli.copilot_auth import _token_fingerprint
+
+        fp1 = _token_fingerprint("gho_abc123")
+        fp2 = _token_fingerprint("gho_xyz789")
+        assert fp1 != fp2
+
+    def test_length(self):
+        from hermes_cli.copilot_auth import _token_fingerprint
+
+        assert len(_token_fingerprint("gho_test")) == 16
+
+
+class TestCallerIntegration:
+    """Test that callers correctly use token exchange."""
+
+    @patch("hermes_cli.copilot_auth.resolve_copilot_token", return_value=("gho_raw", "GH_TOKEN"))
+    @patch("hermes_cli.copilot_auth.get_copilot_api_token", return_value="exchanged_jwt")
+    def test_auth_resolve_uses_exchange(self, mock_exchange, mock_resolve):
+        from hermes_cli.auth import _resolve_api_key_provider_secret
+
+        # Create a minimal pconfig mock
+        pconfig = MagicMock()
+        token, source = _resolve_api_key_provider_secret("copilot", pconfig)
+        assert token == "exchanged_jwt"
+        assert source == "GH_TOKEN"
+        mock_exchange.assert_called_once_with("gho_raw")