fix(desktop): recover sessions after provider removal

[eiritsu]

Summary

When a persisted session references a provider that has since been removed from configuration, Desktop/TUI resume could restore the stale provider identity and endpoint metadata before the model picker became usable.

This change makes stored runtime restoration validate the provider identity against the current provider registry. If the provider is no longer routable, the restored session keeps its model and session-local reasoning/service-tier settings but drops the stale provider, base URL, and API mode. The session can then initialize with the current default provider and be switched through the existing session-scoped model picker.

Scope

• Desktop/TUI stored-session runtime restoration only.
• No changes to custom endpoint CRUD or provider configuration storage.
• No credentials or API keys are persisted or restored.

Tests

• python -m pytest tests/test_tui_gateway_server.py -q — 518 passed
• python -m py_compile tui_gateway/server.py
• git diff --check

[teknium1]

Resolved via PR #97008, which salvaged the heal-or-fallback fix for stale-provider session resume onto current main (based on #96111, with #78947 as the earliest submission of the fallback half — both credited). The Desktop can now recover sessions after provider removal: renamed providers heal to the new identity, deleted ones fall back to the configured default. Thanks for your work on this bug class.