fix(tests): pin _cfg_mtime in _models_with_cfg to prevent ordering-dependent failure — v0.50.109 (fixes #754) (#756)

## Summary

Fixes the ordering-dependent test failure in `test_custom_provider_display_name.py` (issue #754).

**Root cause:** `_models_with_cfg()` patches `config.cfg` then calls `get_available_models()`. That function checks `config.yaml`'s mtime on every call — if it has changed since the last `reload_config()`, it calls `reload_config()` again, which reads from disk and silently overwrites the patch. Any test that writes `config.yaml` (e.g. via `save_settings()`) before this test runs changes the mtime and triggers the reload.

**Fix:** Pin `config._cfg_mtime` to the current `config.yaml` mtime before calling `get_available_models()`, then restore it in the `finally` block. This is the same pattern already used in `test_model_resolver.py` (lines 249, 393).

**Also restores `_cfg_mtime`** in the `finally` block so the patch leaves no side effects on subsequent tests.

## Tests

1510 passed — the previously-flaky test now passes regardless of which tests ran before it.

Closes #754

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.109] — 2026-04-20
+
+### Fixed
+- **Named custom provider test isolation** — `_models_with_cfg()` in `tests/test_custom_provider_display_name.py` now pins `_cfg_mtime` before calling `get_available_models()`, preventing the mtime-guard inside that function from firing `reload_config()` and silently discarding the patched `config.cfg`. This fixes an ordering-dependent test failure where any test that wrote `config.yaml` before this test ran would cause `get_available_models()` to return the real OpenRouter model list instead of the patched Agent37 group. (Fixes #754)
+
 ## [v0.50.108] — 2026-04-20
 
 ### Fixed