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

tests/test_custom_provider_display_name.py

@@ -9,18 +9,33 @@ import api.config as config
 
 
 def _models_with_cfg(model_cfg=None, custom_providers=None, active_provider=None):
-    """Temporarily patch config.cfg, call get_available_models(), restore."""
+    """Temporarily patch config.cfg, call get_available_models(), restore.
+
+    Also pins _cfg_mtime to the current config.yaml mtime before calling
+    get_available_models().  Without this, if a prior test wrote config.yaml
+    (changing its mtime), the mtime-guard inside get_available_models() fires
+    reload_config() which overwrites config.cfg with the real on-disk values,
+    silently discarding the patch and causing ordering-dependent failures.
+    This matches the pattern used in test_model_resolver.py.
+    """
     old_cfg = dict(config.cfg)
+    old_mtime = config._cfg_mtime
     config.cfg.clear()
     if model_cfg:
         config.cfg["model"] = model_cfg
     if custom_providers is not None:
         config.cfg["custom_providers"] = custom_providers
+    # Pin mtime so get_available_models() skips its reload_config() guard.
+    try:
+        config._cfg_mtime = config.Path(config._get_config_path()).stat().st_mtime
+    except Exception:
+        config._cfg_mtime = 0.0  # no config.yaml present; reload guard is a no-op
     try:
         return config.get_available_models()
     finally:
         config.cfg.clear()
         config.cfg.update(old_cfg)
+        config._cfg_mtime = old_mtime
 
 
 # ── Named provider shows its name in the dropdown ─────────────────────────────