fix(session): recover orphan reasoning stream parts

[bvolpato]

Issue for this PR

Closes #36241

Type of change

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

What does this PR do?

AI SDK reports a missing reasoning/text start as an in-band part ... not found error, then keeps streaming. OpenCode promoted that bookkeeping error to a fatal turn failure.

Emit a missing start before orphan deltas or end events, preserve provider metadata, and ignore only matching AI SDK bookkeeping errors. Real provider errors still fail.

Based on #39287 by DragonBaiMo, rebased onto current dev, with coverage moved into current adapter suite. Existing PR no longer passes that suite after recent adapter changes.

How did you verify your code works?

• bun test test/session/llm.test.ts: 29 pass
• bun typecheck from packages/opencode
• pre-push repository typecheck: 30 tasks pass

Screenshots / recordings

Not applicable.

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

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]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Related PRs Found

#39287 - fix(session): repair orphan reasoning/text stream parts instead of failing the turn

• fix(session): repair orphan reasoning/text stream parts instead of failing the turn #39287
• This is the original PR by DragonBaiMo that the current PR is based on. The current PR (fix(session): recover orphan reasoning stream parts #41630) explicitly mentions it was "rebased onto current dev, with coverage moved into current adapter suite" because the existing PR no longer passes the adapter suite after recent changes.

#26167 - fix(session): retry empty stream truncations and discard partial parts

• fix(session): retry empty stream truncations and discard partial parts #26167
• Related to stream part handling and recovery from truncations/partial parts, similar domain.

These are related PRs in the same stream recovery space, but #39287 is the direct predecessor that this PR builds upon.

[github-actions]

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