Fix percent-encoded audio file URIs

[Ehsan-hash]

Summary

Closes #390.

Audio.from_file() accepts file URIs, but currently treats percent escapes as literal filename characters. A WAV named clip one.wav loads through its plain path and fails through Path.as_uri(). AudioURLChunk reaches the same loader.

Changes

• Decode percent escapes once inside the existing file:// branch using urllib.parse.unquote. Plain local paths remain untouched, including filenames containing literal %20.
• Clarify the accepted file-URI input in the method's docstring.
• Add generated-WAV regressions for spaces, literal percent escapes, Unicode, plus signs, unescaped filenames, missing files, and the real AudioURLChunk encoder workflow.

The change adds no dependency or new URL/authority handling. It keeps the existing file-prefix routing and audio decoding behavior.

Verification

Tested on macOS with Python 3.12.15, pinned uv 0.12.6, and the frozen all-extras/development environment. Tests use generated audio and offline model caches.

Unchanged source: new regressions fail in five encoded-URI cases; two controls pass.
With the fix: all seven regressions pass, comparing actual samples and sampling rates.
Existing focused audio and AudioURLChunk checks: 36 passed.
Full CI-shaped non-integration suite: 1,329 passed, 16 skipped.
Source doctests: 53 passed.
Ruff check and format, mypy, lock check, and manual pre-commit hooks: passed.

uv sync --frozen --all-extras --group dev --python 3.12
uv run pytest -q tests/unit/tokens/tokenizers/test_audio_file_uri.py
uv run pytest --cov=mistral_common tests/ --ignore=tests/integrations --ignore=tests/integration -n 2 --dist loadfile
uv run pytest --doctest-modules ./src
uv run ruff check .
uv run ruff format --check .
uv run mypy .
uv lock --check
uv run pre-commit run --files src/mistral_common/tokens/tokenizers/audio.py tests/unit/tokens/tokenizers/test_audio_file_uri.py

• Tests added or updated for changed behavior
• Documentation and public API examples updated where needed
• Ruff, mypy, and pytest checks pass