fix(opencode): handle AbortError in session processor to prevent side…

[patihomirov]

Issue for this PR

Closes #26667

Type of change

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

What does this PR do?

When an LLM stream is interrupted (network timeout, API disconnection), the unhandled AbortError propagates up the Effect.js fiber stack and crashes the entire sidecar process.

In packages/opencode/src/session/processor.ts, Effect.catchCauseIf only catches non-interrupt causes. When AbortError occurs during stream interruption, it bypasses Effect.catch(halt) and crashes the sidecar instead of being handled gracefully.

The fix adds Effect.catchCause before Effect.catch(halt) to properly catch and handle AbortError via the existing halt() handler. This ensures the error is logged, stored in assistantMessage.error, and the session transitions to idle state - instead of crashing the sidecar.

How did you verify your code works?

Built a patched desktop version and ran a long LLM task that triggered a stream interruption. Previously this would crash the sidecar with AbortError: Aborted logged in journalctl. With the patch, the sidecar stays alive and the error is handled gracefully (logged via halt(), session transitions to idle).

Screenshots / recordings

N/A - not a UI change.

Checklist

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

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