fix(tui): surface non-stop finish reasons in assistant header

[truffle-dev]

Issue for this PR

Closes #23928

Type of change

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

What does this PR do?

The assistant header status bar already shows · interrupted when the message ended with MessageAbortedError. When the model itself ends a response abnormally (length limit hit, content filter triggered, error during streaming), message.finish is set to length / content-filter / error, the message is treated as final, and the user sees no indication of why the output stopped.

In #23928 the reporter confirmed via SQL query against part that storage matches the TUI cut-off (text in storage ends at the same character the TUI shows). The stream/storage path is doing the right thing — the model is returning a truncated response and opencode persists exactly what was returned. Surfacing the cause in the header is the missing piece.

Mirrors the existing interrupted indicator pattern in both:

• AssistantMessage TUI status bar (packages/opencode/src/cli/cmd/tui/routes/session/index.tsx)
• formatAssistantHeader markdown transcript export (packages/opencode/src/cli/cmd/tui/util/transcript.ts)

Header examples after this change:

• length: ▣ Plan · MiniMax M2.5 · 41.0s · truncated by length limit
• content-filter: ▣ Build · gpt-4o · 12.3s · stopped by content filter
• error: ▣ Build · claude-3-5-sonnet · 5.4s · ended with error
• stop: ▣ Build · claude-3-5-sonnet · 5.4s (unchanged)

How did you verify your code works?

• bun test packages/opencode/test/cli/tui/transcript.test.ts — 29/29 pass (13 new cases covering length/content-filter/error/stop/tool-calls/other/unknown/undefined plus the metadata-disabled path)
• bun test packages/opencode — 150/150 pass
• bun typecheck clean
• Verified the existing interrupted indicator path is unchanged (same <Show> block above the new one)

Screenshots / recordings

Text-only TUI change — captured in the four header examples above.

Checklist

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

[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]

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

[truffle-dev]

Just checking in on this one. Eight days since open, still mergeable, all CI green against current dev. Happy to rebase if anything's drifted. Is there anything else needed before this can be merged?

[github-actions]

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