fix: stop empty unknown provider response loop

[tita-n]

Issue for this PR

Closes #32366

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When a provider returns an empty response with an unknown finish reason, the session prompt loop can continue creating new assistant turns indefinitely. The session remains busy, leaving the UI stuck on “thinking” without displaying an error or recovering to an idle state.

This PR checks the completed assistant turn before continuing the prompt loop. If the finish reason is unknown and the turn contains no text, reasoning, or tool output, it records a Provider returned an empty response error, publishes the session error, and returns the session to an idle state.

Turns that contain output continue through the existing path, so the change is limited to genuinely empty provider responses.

A deterministic test-provider response and a CLI regression test were added for this case.

How did you verify your code works?

I tested the changes locally with:

• bun typecheck
• bun test --timeout 30000 test/session/processor-effect.test.ts
• bun test --timeout 30000 test/cli/run/stream.transport.test.ts
• The new empty-response CLI regression test

The regression test confirms that an empty unknown provider response exits promptly with a nonzero status and reports the expected error.

Screenshots / recordings

Not applicable; this is a backend/CLI behavior change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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