feat(coding-agent): allow thinking renderers to replace blocks

[shoucandanghehe]

Summary

Rerolls the assistant thinking renderer replacement API for #1728 against current main.

Thinking renderers now support:

• Component: legacy append behavior below the default thinking Markdown.
• { type: "append", component }: explicit append behavior.
• { type: "replace", component }: suppresses the default thinking Markdown for that thinking block and renders the replacement component instead.

Details

• Passes parent assistant metadata and provider thinking content metadata to renderers so async/stateful renderers can build stable keys without colliding across transcript history.
• Keeps legacy component returns compatible by detecting TUI components before structured result objects.
• Uses first-replacement-wins when multiple renderers try to replace the same thinking block, while preserving append renderers after replacements.
• Preserves requestRender() semantics for both mounted async components and late undefined -> append/replace renderer output.
• Keeps replacement-capable visible thinking blocks in the repaintable live region so late async replacement cannot leave stale raw thinking rows in native scrollback.
• Updates extension docs and the coding-agent changelog.

Verification

bun test packages/coding-agent/test/modes/components/transcript-container.test.ts packages/coding-agent/test/modes/components/assistant-message-mermaid.test.ts
bun --cwd=packages/coding-agent run check:types
bun --cwd=packages/coding-agent run check:docs

Results:

• 60 pass, 0 fail, 182 expect() calls
• check:types passed
• check:docs passed (Docs index fresh for 121 docs)

Closes #1728.

Previous stale PR: #2340.

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[roboomp]

P2: coherent implementation of the #1728 extension API shape, but it expands a public rendering contract, so this still needs the maintainer’s explicit merge call.

One should-fix: the packages/coding-agent/CHANGELOG.md entry is under released 15.11.3 instead of ## [Unreleased].

Verified locally: focused component tests passed (60 pass), bun --cwd=packages/coding-agent run check:types passed, and check:docs passed. Thanks for rerolling this against current main.