test: speed up slow tests (backoff + subprocess + IMDS network) (#11797)

Cuts shard-3 local runtime in half by neutralizing real wall-clock
waits across three classes of slow test:

## 1. Retry backoff mocks

- tests/run_agent/conftest.py (NEW): autouse fixture mocks
  jittered_backoff to 0.0 so the `while time.time() < sleep_end`
  busy-loop exits immediately. No global time.sleep mock (would
  break threading tests).
- test_anthropic_error_handling, test_413_compression,
  test_run_agent_codex_responses, test_fallback_model: per-file
  fixtures mock time.sleep / asyncio.sleep for retry / compression
  paths.
- test_retaindb_plugin: cap the retaindb module's bound time.sleep
  to 0.05s via a per-test shim (background writer-thread retries
  sleep 2s after errors; tests don't care about exact duration).
  Plus replace arbitrary time.sleep(N) waits with short polling
  loops bounded by deadline.

## 2. Subprocess sleeps in production code

- test_update_gateway_restart: mock time.sleep. Production code
  does time.sleep(3) after `systemctl restart` to verify the
  service survived. Tests mock subprocess.run \u2014 nothing actually
  restarts \u2014 so the wait is dead time.

## 3. Network / IMDS timeouts (biggest single win)

- tests/conftest.py: add AWS_EC2_METADATA_DISABLED=true plus
  AWS_METADATA_SERVICE_TIMEOUT=1 and ATTEMPTS=1. boto3 falls back
  to IMDS (169.254.169.254) when no AWS creds are set. Any test
  hitting has_aws_credentials() / resolve_aws_auth_env_var() (e.g.
  test_status, test_setup_copilot_acp, anything that touches
  provider auto-detect) burned ~2-4s waiting for that to time out.
- test_exit_cleanup_interrupt: explicitly mock
  resolve_runtime_provider which was doing real network auto-detect
  (~4s). Tests don't care about provider resolution \u2014 the agent
  is already mocked.
- test_timezone: collapse the 3-test "TZ env in subprocess" suite
  into 2 tests by checking both injection AND no-leak in the same
  subprocess spawn (was 3 \u00d7 3.2s, now 2 \u00d7 4s).

## Validation

| Test | Before | After |
|---|---|---|
| test_anthropic_error_handling (8 tests) | ~80s | ~15s |
| test_413_compression (14 tests) | ~18s | 2.3s |
| test_retaindb_plugin (67 tests) | ~13s | 1.3s |
| test_status_includes_tavily_key | 4.0s | 0.05s |
| test_setup_copilot_acp_skips_same_provider_pool_step | 8.0s | 0.26s |
| test_update_gateway_restart (5 tests) | ~18s total | ~0.35s total |
| test_exit_cleanup_interrupt (2 tests) | 8s | 1.5s |
| **Matrix shard 3 local** | **108s** | **50s** |

No behavioral contract changed \u2014 tests still verify retry happens,
service restart logic runs, etc.; they just don't burn real seconds
waiting for it.

Supersedes PR #11779 (those changes are included here).

tests/test_timezone.py

@@ -159,18 +159,34 @@ class TestCodeExecutionTZ:
         return _json.dumps({"error": f"unexpected tool call: {function_name}"})
 
     def test_tz_injected_when_configured(self):
-        """When HERMES_TIMEZONE is set, child process sees TZ env var."""
+        """When HERMES_TIMEZONE is set, child process sees TZ env var.
+
+        Verified alongside leak-prevention + empty-TZ handling in one
+        subprocess call so we don't pay 3x the subprocess startup cost
+        (each execute_code spawns a real Python subprocess ~3s).
+        """
         import json as _json
         os.environ["HERMES_TIMEZONE"] = "Asia/Kolkata"
 
+        # One subprocess, three things checked:
+        #   1) TZ is injected as "Asia/Kolkata"
+        #   2) HERMES_TIMEZONE itself does NOT leak into the child env
+        probe = (
+            'import os; '
+            'print("TZ=" + os.environ.get("TZ", "NOT_SET")); '
+            'print("HERMES_TIMEZONE=" + os.environ.get("HERMES_TIMEZONE", "NOT_SET"))'
+        )
         with patch("model_tools.handle_function_call", side_effect=self._mock_handle):
             result = _json.loads(self._execute_code(
-                code='import os; print(os.environ.get("TZ", "NOT_SET"))',
-                task_id="tz-test",
+                code=probe,
+                task_id="tz-combined-test",
                 enabled_tools=[],
             ))
         assert result["status"] == "success"
-        assert "Asia/Kolkata" in result["output"]
+        assert "TZ=Asia/Kolkata" in result["output"]
+        assert "HERMES_TIMEZONE=NOT_SET" in result["output"], (
+            "HERMES_TIMEZONE should not leak into child env (only TZ)"
+        )
 
     def test_tz_not_injected_when_empty(self):
         """When HERMES_TIMEZONE is not set, child process has no TZ."""
@@ -186,20 +202,6 @@ class TestCodeExecutionTZ:
         assert result["status"] == "success"
         assert "NOT_SET" in result["output"]
 
-    def test_hermes_timezone_not_leaked_to_child(self):
-        """HERMES_TIMEZONE itself must NOT appear in child env (only TZ)."""
-        import json as _json
-        os.environ["HERMES_TIMEZONE"] = "Asia/Kolkata"
-
-        with patch("model_tools.handle_function_call", side_effect=self._mock_handle):
-            result = _json.loads(self._execute_code(
-                code='import os; print(os.environ.get("HERMES_TIMEZONE", "NOT_SET"))',
-                task_id="tz-leak-test",
-                enabled_tools=[],
-            ))
-        assert result["status"] == "success"
-        assert "NOT_SET" in result["output"]
-
 
 # =========================================================================
 # Cron timezone-aware scheduling