refactor(gmi): move User-Agent to profile.default_headers

The previous revision of this PR added six GMI-specific branches
(`elif base_url_host_matches(..., 'api.gmi-serving.com')`) across
run_agent.py and agent/auxiliary_client.py, plus a _HERMES_UA_HEADERS
constant in auxiliary_client.py.

ProviderProfile already has a `default_headers: dict[str, str]` field
commented as 'Client-level quirks (set once at client construction)'.
Other plugins (ai-gateway, kimi-coding) already use it. Two of the four
auxiliary_client sites we previously patched already had a generic
`else: profile.default_headers` fallback that picked it up (so did
both run_agent sites).

This revision:

* Sets `default_headers={'User-Agent': 'HermesAgent/<ver>'}` on the
  GMI profile in plugins/model-providers/gmi/__init__.py.
* Reverts all six GMI-specific branches in run_agent.py and
  auxiliary_client.py.
* Adds the generic profile-fallback `else` block to the two
  auxiliary_client sites (`_to_async_client`, `resolve_provider_client`)
  that didn't have it yet. This benefits every provider whose profile
  declares default_headers, not just GMI — e.g. Vercel AI Gateway's
  HTTP-Referer/X-Title now flow through the async client path too.
* Replaces the GMI-specific URL-branch tests with a profile-level
  assertion and keeps the run_agent integration test (with
  `provider='gmi'` so the fallback picks up the profile).

Net diff vs main: +82/-0 across 5 files, touching only the GMI plugin,
two generic fallback blocks in auxiliary_client.py, AUTHOR_MAP, and
tests. No core files change.

Based on #20907 by @isaachuangGMICLOUD.

tests/hermes_cli/test_gmi_provider.py

@@ -284,6 +284,22 @@ class TestGmiAuxiliary:
         assert model == "google/gemini-3.1-flash-lite-preview"
         assert mock_openai.call_args.kwargs["api_key"] == "gmi-test-key"
         assert mock_openai.call_args.kwargs["base_url"] == "https://api.gmi-serving.com/v1"
+        # GMI profile declares default_headers with a HermesAgent User-Agent
+        # for traffic attribution. The generic profile-fallback branch in
+        # resolve_provider_client should carry it through to the OpenAI client.
+        headers = mock_openai.call_args.kwargs.get("default_headers", {})
+        assert headers.get("User-Agent", "").startswith("HermesAgent/")
+
+    def test_gmi_profile_declares_hermes_user_agent(self):
+        """The GMI plugin sets a HermesAgent/<ver> User-Agent on its profile."""
+        from providers import get_provider_profile
+
+        profile = get_provider_profile("gmi")
+        assert profile is not None
+        ua = profile.default_headers.get("User-Agent", "")
+        assert ua.startswith("HermesAgent/"), (
+            f"expected GMI profile User-Agent to start with 'HermesAgent/', got {ua!r}"
+        )
 
     def test_resolve_provider_client_accepts_gmi_alias(self, monkeypatch):
         monkeypatch.setenv("GMI_API_KEY", "gmi-test-key")