fix(opencode): no response handling

[SoShymKing]

Issue for this PR

Fixes #17717
Addresses the bug described in #17717.
This is a template-compliant follow-up to #17719, which attempted the same fix but was automatically closed for not using the required PR template.

Type of change

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

What does this PR do?

This addresses the unexpected abort and no-response behavior reported in #17717 and previously attempted in #17719.

When the model or provider raises an abort while the session itself has not been cancelled, OpenCode now treats that case as a transient provider/client-side abort rather than a user cancellation.
These errors are routed through the existing retry and backoff path, while explicit user or session cancellations still terminate immediately without retrying.

The change also improves handling for stalled post-tool continuations and empty assistant streams.
OpenCode now detects post-tool continuation failures after completed tool calls, converts no-response cases into typed errors with diagnostics, and retries blank or structural-only assistant turns instead of persisting them as successful empty responses.

How did you verify your code works?

Ran from 'packages/opencode':
'''bash
bun test test/session/retry.test.ts test/session/message-v2.test.ts test/session/processor-effect.test.ts
bun typecheck
'''

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

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

If you do not follow this template your PR will be automatically rejected.

[github-actions]

Thanks for your contribution!

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Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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