fix(tui): preserve fallback provider during refresh

[xushun007]

Related Issue

None. This is a focused, reproducible bug fix found while reviewing the current main branch.

Problem

Provider model refreshes use a remove-then-set flow so stale model aliases can be deleted. Removing a provider also clears defaultProvider when it points to that provider.

The managed OAuth, open-platform, and custom-registry refresh branches rebuilt the provider successfully but omitted defaultProvider from the complete config patch. On the v2 atomic persistence path, that left the staged config without the configured fallback. A later provider refresh could then persist the missing value and silently remove a still-valid fallback provider.

What changed

• Include the rebuilt defaultProvider value in every complete provider-refresh config patch.
• Preserve the fallback when a provider is refreshed and re-added.
• Continue clearing the fallback when a registry genuinely removes its provider, because the rebuilt config then explicitly carries undefined.
• Update the refresh test host to model the real removal cascade and add a regression test for managed OAuth refresh.
• Add a patch changeset for the CLI.

Validation

• pnpm exec vitest run apps/kimi-code/test/tui/utils/refresh-providers.test.ts apps/kimi-code/test/tui/kimi-tui-startup.test.ts — 74 tests passed.
• pnpm run typecheck — passed across all packages and applications.
• pnpm run lint — passed with no errors (existing repository warnings remain).
• git diff --check and changeset status validation — passed.
• The full repository test suite was attempted, but the local environment hit unrelated image-compression/network timeouts and EMFILE: too many open files; the affected provider-refresh suites passed independently.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: 63677cd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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