## Summary
Regression fix for #751.
Models with custom or unrecognized prefixes (e.g. `custom-provider/my-model`, `test/import-model`) were being incorrectly replaced with the active provider default. Root cause: `_normalize_provider_id("custom-provider")` matched the `"custom"` prefix and returned `"custom"`, which ≠ `active_provider` → normalization fired.
Two-part fix:
1. Add `"custom"` and `"openrouter"` to the `model_provider` exclusion set in `_resolve_compatible_session_model` (parallel to the existing `active_provider` guard)
2. Return `""` for unknown prefixes in `_normalize_provider_id` so the `if model_provider` truthiness check safely short-circuits
Adds a regression test covering `custom-provider/`, `test/`, `my-local-llm/`, and `lmstudio-community/` prefixes.
## Tests
1499 passed, 0 failures (was 2 failures before this fix)
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