harden #5979 per Codex gate: union extra_models bucket + profile-isolation fail-safe

Codex regression gate found two real issues on the first cut:

1. extra_models bucket: a provider's advertised ids can be split across the
   catalog's 'models' (visible) and 'extra_models' (picker overflow) buckets.
   Reading only 'models' could miss a bare id that overflowed, leaving the
   #433 stale prefix unstripped. _endpoint_advertised_model_ids now unions
   both buckets.

2. Profile isolation (profiles are islands): _available_models_cache is a
   process global, so a concurrently-active foreign profile could publish the
   snapshot a different profile then reads for provenance — potentially
   stripping an advertised full id against another profile's catalog. The
   accessor now rejects the snapshot unless its published source fingerprint
   (whose config_yaml axis is the profile-specific config path) matches the
   current runtime fingerprint; any mismatch fails safe to verbatim preserve.

Adds tests: extra_models-counts-as-advertised (#433 via overflow bucket) and
foreign-profile-catalog-preserves-verbatim (isolation fail-safe). Test helpers
now stamp the source fingerprint exactly as the real publish sites do.

tests/test_sprint40_ui_polish.py

@@ -156,6 +156,7 @@ class TestCustomEndpointModelStripping:
         old_cfg = dict(_api_config.cfg)
         old_cache = _api_config._available_models_cache
         old_memo = _api_config._advertised_model_ids_memo
+        old_fp = _api_config._available_models_cache_source_fingerprint
         model_cfg = {}
         if provider:
             model_cfg['provider'] = provider
@@ -171,6 +172,7 @@ class TestCustomEndpointModelStripping:
                     'models': [{'id': mid, 'label': mid} for mid in advertised_ids],
                 }]
             }
+            _api_config._available_models_cache_source_fingerprint = _api_config._models_cache_source_fingerprint()
         _api_config._advertised_model_ids_memo = None
         try:
             return _api_config.resolve_model_provider(model_id)
@@ -179,6 +181,7 @@ class TestCustomEndpointModelStripping:
             _api_config.cfg.update(old_cfg)
             _api_config._available_models_cache = old_cache
             _api_config._advertised_model_ids_memo = old_memo
+            _api_config._available_models_cache_source_fingerprint = old_fp
 
     def test_prefixed_model_stripped_for_custom_endpoint(self):
         """Issue #433: 'openai/gpt-5.4' with custom base_url returns bare 'gpt-5.4'