fix(javascript): discover TypeScript tests without expanding source scope

[daBOB]

Problem

A JavaScript scan discovers production .js/.jsx/.mjs/.cjs files but silently misses tests written in TypeScript. The JavaScript plugin leaves test_file_extensions at the production-extension default, so normal test/*.test.ts suites do not contribute to test mapping.

An additional boundary affects scans from the project root: the supplemental discovery helper skips test directories beneath the scanned path, assuming the source finder already saw their files. That assumption fails when test extensions differ from production extensions.

Change

• Configure JavaScript test discovery to accept .ts, .tsx, .mts and .cts alongside the existing JavaScript extensions. Production extensions remain unchanged.
• In standard/configured test directories beneath the scan root, collect only test extensions omitted by production discovery. Already-scanned JavaScript tests retain their existing path and are not added again.
• Keep import resolution and test inference unchanged; the existing hooks resolve these tests against the JavaScript production set.

Validation

• 32 real production-phase cases cover eight test extensions with relative/absolute subdirectory and project-root scans. They verify successful import mapping, JavaScript-only production scope, excluded TypeScript context, and an unrelated JavaScript file that remains untested.
• The initial external-TypeScript cases failed before plugin configuration (8 failed / 8 JavaScript controls passed). The project-root TypeScript cases then failed before the shared-helper correction (8 failed / 24 passed).
• Focused discovery tests: 37 passed. JavaScript/shared framework suite: 538 passed, 2 skipped. Ruff and whitespace checks pass.
• Read-only verification against a real browser workspace, with fix: preserve dependency edges for relative tree-sitter file paths #781's path normalization held constant: the same 55 JavaScript production files and 132 graph edges; external candidates increase from 0 to 117 TypeScript files (111 named tests plus 6 support files); test-gap findings change from 50 to 17. Source and scanner-state hashes are unchanged. Remaining findings require normal adjudication; these counts demonstrate discovery, not whether every behavior is tested.

The validation checkout combines this patch with #777, #778, #781 and #782; those other changes are not included in this PR. Full combined suite: 6932 passed, 23 skipped, 2 failed. Both failures are the duplicated historical test_do_run_batches_dry_run_generates_packet_and_prompts cases, independently reproduced on pristine base 3a7735d.

Existing limitations remain: discovery uses the configured test directories, and dynamically constructed imports, query-suffixed imports and CommonJS patterns retain their current mapping behavior. No application code, scanner dispositions or installed CLI was changed to produce the verification.