fix(core): don't warn when tree-sitter highlight fails during client destroy

[HARIOM-JHA01]

Fixes #1319

Problem

When @opentui/core is torn down, TreeSitterClient.destroy() rejects any in-flight highlightOnce() promises with a generic Error("TreeSitter client destroyed"). CodeRenderable.startHighlight() treats every rejection as a real failure and logs a misleading warning:

Code highlighting failed, falling back to plain text: Error: ...

This races with client teardown on exit, so opencode users see the warning unexpectedly on shutdown even though nothing is actually broken.

Reported in anomalyco/opencode#36454.

Fix

• client.ts: introduce a typed TreeSitterClientDestroyedError and reject pending requests (and pending initialization) with it in destroy().
• Code.ts: silently bail out when highlighting fails due to client destroy (or the renderable is already destroyed), preserving warnings for genuine worker failures.
• Testing: mock-tree-sitter-client supports rejectHighlightOnce; client destroy tests now assert TreeSitterClientDestroyedError;
• Code renderer tests verify no console.warn on destroy-triggered failures while a real Error still warns.

Tests

• bun test packages/core/src/lib/tree-sitter/client.test.ts (49 pass)
• oxlint / oxfmt clean
• Renderer tests (Code.test.ts) require the native lib and run in CI.