Update Codex provider banner when Codex path settings change

[jamesx0416]

What Changed

• added a typed server.validateCodexCli RPC so the server can revalidate Codex availability for an effective binaryPath / homePath pair
• changed provider health from a startup-only snapshot to a mutable in-memory snapshot so server.getConfig() and the existing Codex banner can reflect revalidation results
• triggered Codex validation from the Settings page when the codexBinaryPath or codexHomePath input blurs
• refreshed the same provider status after Codex session start success/failure so the banner self-corrects from real launch outcomes
• added focused server and browser regression coverage for the new validation path

Why

The existing banner only reflected a startup-time codex-from-PATH probe, so changing Codex path settings could leave a stale "not on PATH" error visible even when the configured Codex binary worked. This keeps the banner tied to real validation results for the current Codex settings without weakening the existing session-start hard gate.

Closes

• Closes Healthcheck appears to not be using the custom CODEX_HOME and path binary settings #693

• Closes [Bug]: CODEX_HOME path override is ignored in some parts of the app #1158

• Closes Codex binary path stuck on stale value; app keeps checking /Users/username/bin/codex even after reset #1022

• Closes Codex provider status following installation can't be dismissed without closing and re-opening app #197

• Closes Persistent "Codex CLI not found" error after installation of codex CLI #372

• Should close 'codex' not found error but it actually works #630

UI Changes

Small interaction change only:

• leaving either Codex path field in Settings now revalidates Codex and updates the existing provider banner/status

No screenshots or video attached.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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