fix: validate download path containment in runner

[markbackman]

Summary

• Fixes a path traversal in the development runner's /files/{filename:path} download endpoint, where %2F-encoded separators bypassed Starlette's path normalisation and let an unauthenticated request read any file the pipecat process could access.
• The endpoint now resolves the joined path and rejects any filename that escapes --folder with a 403, and returns 404 (instead of an implicit null 200) when --folder is unset.
• Adds tests covering literal ../, %2F-decoded traversal, absolute paths, and symlink escape.

Testing

• uv run pytest tests/test_runner_downloads.py
• Manual: start the runner with --folder /tmp/bot_media, confirm legitimate files still download and that curl http://127.0.0.1:7860/files/..%2F..%2Fetc%2Fpasswd returns 403.

[codecov]

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pipecat/runner/run.py	54.54%	5 Missing ⚠️

Files with missing lines	Coverage Δ
src/pipecat/runner/run.py	11.35% <54.54%> (+11.35%)	⬆️

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[filipi87]

Nice improvement! 🚀