Adds chat_completions and transcription as SDK function columns, backed
by the existing rust_bridge test files. Adds a fourth strategy folder,
existing_e2e_test_sdk, that points at already-existing live-API SDK
tests (tests/ocr_tests/ as a whole folder, plus chat completion and
Whisper transcription tests) instead of writing new parity tests.
Extends selector_matches_node with trailing-slash folder selectors so
a whole test folder can back one matrix cell.
