fix: normalize stale session models after provider switch — v0.50.99 (#751)

## Summary

Rebased-on-behalf of @likawa3b (originally PR #748 — stale base).

Sessions can outlive provider changes. When an old session still points to a model from a previous provider (e.g. `gemini-3.1-pro-preview` after switching the agent to OpenAI Codex), starting a chat hits the wrong backend and fails silently.

This PR adds a lightweight normalization pass:
- `_normalize_provider_id()` maps common prefixes to canonical provider IDs
- `_resolve_compatible_session_model()` checks the session model's provider against `active_provider` and returns the default model if they differ
- `_normalize_session_model_in_place()` is called at GET `/api/session` — corrects and persists stale models once
- Chat start also normalizes via `_resolve_compatible_session_model()` and returns `effective_model` in the response
- `messages.js` applies `effective_model` back to the UI/localStorage/dropdown if set

Closes #748

## Tests

1498 passed (2 pre-existing ordering failures unrelated to this PR; 5 new tests added in `test_provider_mismatch.py`).

**Original author:** @likawa3b

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Hermes Web UI -- Changelog
 
+## [v0.50.99] — 2026-04-20
+
+### Fixed
+- **Stale session models normalized after provider switch** — sessions that still reference a model from a previous provider (e.g. a `gemini-*` model after switching to OpenAI Codex) are silently corrected to the current provider's default on load, preventing startup failures. (Closes #748, credit: @likawa3b)
+
 ## [v0.50.98] — 2026-04-20
 
 ### Fixed