fix(web): default provider selection for users without Codex

[mfazekas]

What Changed

• Command palette project creation no longer hard-codes codex as the new project's defaultModelSelection. It resolves the first usable provider instance (ready → non-error → enabled, then the codex literal), stamps that instance's own default model, and adds the missing providers entry to the callback dependencies so the handler does not act on the at-mount provider snapshot.
• The composer's instance resolution falls through to an enabled instance when the persisted instance id is known-but-disabled — but only when an enabled instance satisfies the thread's driver/continuation lock; otherwise the id is kept so the accurate "provider disabled" server error surfaces. Unknown ids (e.g. a custom instance not yet hydrated) are still returned as before. The composer's driver kind now follows the resolved instance, so options and send context stay consistent with the instance that actually runs.
• resolveSelectableProviderInstance tiers its fallback by probe status and getDefaultProviderInstanceModel provides an instance-scoped default model. Unit tests cover the tiering, requested-instance retention, and the model helper.

Why

Fixes #4116. New users without Codex get their first palette-created project defaulted to codex, so the first thread fails (Failed to spawn Codex App Server process on fresh settings, or Provider instance 'codex' is disabled in T3 Code settings. when codex is disabled) and the thread stays permanently pinned to codex. The composer change also lets existing broken threads recover: on the next send they fall through to a working provider.

UI Changes

Before (thread pinned to codex, turn fails, composer shows the OpenAI icon with a claude model):

After (same flow defaults to the ready Claude instance and the send succeeds):

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

Note

Fix default provider selection in command palette and composer for users without Codex

• Replaces the hardcoded codex/DEFAULT_MODEL fallback in CommandPalette.tsx with logic that picks the best available provider instance and its default model using resolveSelectableProviderInstance and getDefaultProviderInstanceModel.
• Adds getDefaultProviderInstanceModel to providerInstances.ts, which prefers the first non-custom model for an instance, then the first listed model, then the driver-level default.
• Improves resolveSelectableProviderInstance to rank instances by status: ready first, then non-error (e.g. warning), then any enabled instance — instead of picking arbitrarily.
• In ChatComposer.tsx, retains an explicitly selected but disabled instance unless a compatible enabled replacement exists, so sends fail explicitly rather than silently switching drivers.
• Behavioral Change: projects created from the command palette and composer turns now use the best available provider rather than always defaulting to codex.

^{Macroscope summarized caf8c05.}

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[macroscopeapp]

Approvability

Verdict: Approved

Bug fix with limited scope to provider selection fallback logic. Changes ensure working providers are preferred over errored ones when user doesn't have Codex. Well-tested with clear intent and self-contained to provider selection utilities.

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[mfazekas]

Pushed a follow-up commit addressing the review findings:

• Bugbot high (composer driver lags instance fallthrough): the composer now derives its driver kind from the resolved instance entry, so options and send context follow the instance that actually runs.
• Bugbot medium (default model ignores resolved instance): project creation now uses an instance-scoped model lookup (getDefaultProviderInstanceModel) instead of the driver default.
• Additionally, resolveSelectableProviderInstance falls back ready → non-error → enabled (an errored codex probe could previously still win by list order during the startup window), and the composer only abandons a known-disabled persisted instance when an enabled instance satisfies the thread's driver/continuation lock — otherwise it keeps the id so the accurate "provider disabled" error surfaces instead of a confusing driver-switch rejection.

Added unit tests for the fallback tiering, requested-instance retention, and the model helper.