fix(plugin): dedupe final thinking snapshots

[qyhfrank]

What

• fix non-stream extractCompletionFromStream() so thinking partial deltas followed by a final accumulated event do not get duplicated
• export and cover extractCompletionFromStream() with focused unit tests
• include the new regression test in test:ci:unit

Why

The streaming converters already avoid duplicating final accumulated thinking frames after timestamped partial deltas, but the non-stream aggregation path in src/plugin.ts still appended the final accumulated thinking text unconditionally. That left /chat/completions non-stream responses inconsistent with the streaming path and could duplicate reasoning text.

Verification

• bun test tests/unit/plugin-stream-extraction.test.ts
• bun run test:ci:unit
• bun run build