fix(chat): preserve text around stray think-close after a complete block

[skyzhao1223]

Description

stripThinkTags (used by MessageText, autoTitle, getLastAssistantText and turnCopy to clean AI replies before rendering) had a destructive bug: its MiniMax step deleted everything up to the first orphaned closing think tag, even when the message contained a complete <thinking>…</thinking> block plus a stray trailing close.

Example: START <thinking>secret</thinking> MIDDLE </thinking> END was stripped to " END", silently losing the legitimate START and MIDDLE text from the rendered message. That combination (a balanced block followed by a leftover close tag from concatenated tool-call output) is exactly the case Step 4 was documented to handle by removing "just the tags".

This fix:

1. Removes complete think/thinking blocks innermost-first in a loop, so nested blocks are fully consumed.
2. Applies the aggressive MiniMax leading-content strip only when no complete block was present; otherwise the orphaned close is removed by Step 4 while surrounding content is preserved.

Related Issues

No existing issue tracked this; discovered during code review of thinkTagFilter.ts.

Type of Change

• fix — Bug fix (non-breaking change which fixes an issue)
• feat — New feature (non-breaking change which adds functionality)
• perf — Performance improvement
• refactor — Code restructuring (no behavior change)
• Breaking change (fix or feature that would break existing functionality)
• docs — Documentation update

Atomic PR Checklist (Rule 1)

• This PR contains exactly one feature or bug fix that cannot be further decomposed
• The PR title follows Conventional Commit format: <type>(<scope>): <subject> (English)

Local Checks (Rule 3)

• bun run format — formatting passes
• bun run lint — no lint errors (skip if no .ts/.tsx changed)
• bunx tsc --noEmit — no type errors (skip if no .ts/.tsx changed)
• bunx vitest run — tests pass
• i18n validated (bun run i18n:types + node scripts/check-i18n.js) — only if src/renderer/, locales/, or src/common/config/i18n/ changed; N/A otherwise
• New/changed user-facing text uses i18n keys (no hardcoded strings)

Runtime Verification

• Verified on macOS
• Verified on Windows
• Verified on Linux
• I have performed a self-review of my own code

Screenshots

N/A (pure string-processing change, covered by unit tests).

Additional Context

• Regression tests added in tests/unit/renderer/utils/thinkTagFilter.test.ts for both the "complete block + stray close" and the standalone-orphan-close cases.
• Verified the bug and the fix directly: the old implementation returned " END" for the input above, the new one returns "START MIDDLE END".
• Note: bunx tsc --noEmit locally reports one pre-existing error (Cannot find module 'mermaid') that also occurs on a clean checkout of main — it is an environment/dependency issue unrelated to this change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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