Update download-files deprecation message

[bcherry]

Updates the deprecation warning and console message for the download-files command when invoked via the agent CLI entrypoint.

Changes

• Updated the deprecation message to be more specific about the version (1.5.10) when this deprecation was introduced
• Clarified the recommended approach: running the command directly against the livekit.agents module instead of through the agent entrypoint
• Updated example command from python -m livekit.agents download-files to uv run -m livekit.agents download-files to reflect current tooling recommendations
• Applied the same message updates to both the console output and the DeprecationWarning

Rationale

The new messaging is clearer about the deprecation timeline and provides a more practical example using modern Python tooling (uv), making it easier for users to understand and adopt the recommended approach.

https://claude.ai/code/session_01Foiz3HektpTey3FaG841Yd

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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