fix(session): handle type validation errors gracefully

[Haohao-end]

Issue for this PR

Closes #31324

Type of change

• Bug fix

What does this PR do?

When the AI SDK receives a non-standard response chunk (e.g. from MCP-related tool calls), it emits a TypeValidationError (name: "AI_TypeValidationError"). The previous code called Effect.fail(event.error) which crashed the stream, leaving the assistant message in a broken state.

Two changes:

1. ai-sdk.ts: TypeValidationError errors are now converted to LLMEvent.providerError events instead of failing the stream. Other error types (e.g. APICallError) still use Effect.fail so retry/classification logic is unaffected.

2. processor.ts: The provider-error handler now checks the retryable flag. Retryable errors still throw to trigger the retry policy. Non-retryable errors call halt() which flushes partial fragments, sets the error on the assistant message, publishes the error event, and sets session status to idle — the same graceful shutdown path used for other terminal errors.

How did you verify your code works?

• bun typecheck — passes
• bun test test/session/ — 362 pass, 0 fail
• All 15 processor-effect.test.ts tests pass (including retry, context-overflow, and error-classification tests)
• All 26 llm.test.ts tests pass

Screenshots / recordings

N/A

Checklist

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