fix(providers): preserve unchanged model caches during sync

[indexGin]

Provider configuration sync currently discards cached model clients for every provider, even when only one provider changed. This preserves the clients for providers whose configuration is unchanged, while refreshing changed providers and removing deleted providers as before.

The regression cases cover unchanged providers, updates to connection and model settings, and provider removal using real configuration and cache objects, without adding mocks or making external provider calls. The eight regression cases failed before the original fix.

Validation after merging develop through 99e479a6 (1.0.2b8):

• Full make all at cc687bdd: 4,609 passed, 211 skipped; formatting, lint and mypy passed.
• The subsequent upstream commit 99e479a6 changes only the PWA update prompt and CHANGELOG. The backend, tests and dependencies are unchanged from that full run.
• Final normal pre-commit hook: 331 passed; dashboard build passed. Hooks were not bypassed.
• The CHANGELOG conflict is resolved while retaining the release notes. The PR still changes only the original three files relative to this develop baseline.

The upstream GitHub Actions run still requires maintainer approval; local results are not a substitute for that run.

AI assistance: Codex investigated, implemented and validated this change.

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