fix(upload): align local gate with 43 supported extensions

[insane66613]

Summary

Align the local NotebookLM file-upload admission gate with the current 43-extension provider contract.

• adds one canonical SUPPORTED_FILE_EXTENSIONS registry
• makes SourceMixin.add_file use the canonical registry with file_path.suffix.lower()
• expands transient media processing classification without narrowing admission
• updates MCP, CLI, skill, and implementation documentation
• adds parameterized positive, mixed-case, negative, media-classification, and drift tests
• preserves provider-side processing failures as ingestion errors rather than local unsupported-format errors

Validation

• uv run pytest tests/test_file_upload.py -q — 73 passed, 1 skipped
• focused Ruff checks — passed
• full suite — 1,348 passed, 38 skipped, 1 deselected; one unrelated Windows home-directory test fails identically on unmodified main

Pre-existing failing test:

tests/cli/test_setup_claude_desktop.py::TestClaudeDesktopConfigPath::test_windows_path_falls_back_when_appdata_is_missing

The failure occurs because the test clears the environment and Path.home() cannot resolve on this Windows host. It reproduces on upstream commit e7874c5 without this patch.