fix(tool): prevent TypeScript files from being misidentified as video

[xiaoju111a]

Summary

Fix TypeScript files (.ts, .tsx, .mts, .cts) being misidentified as MPEG Transport Stream video files, which caused ReadFile to return VideoURLPart and trigger video_in capability check failure.

Resolves #601

Problem

Python's mimetypes.guess_type() maps .ts extension to video/mp2t (MPEG Transport Stream), not TypeScript. This caused:

1. detect_file_type("app.ts") returns kind="video"
2. ReadFile wraps content as VideoURLPart
3. Model capability check fails if video_in is not enabled

Solution

Override the default mimetypes mapping by registering TypeScript extensions with text/typescript MIME type in _EXTRA_MIME_TYPES.

Changes

• src/kimi_cli/tools/file/utils.py: Add .ts, .tsx, .mts, .cts to _EXTRA_MIME_TYPES
• tests/test_file_utils.py: Add test cases for TypeScript file detection