fix(cli): handle rich logging with piped output

[nightcityblade]

Fixes #4552

Summary

• keep the CLI console in forced-terminal mode so colored logs are preserved when output is piped through tools like tee
• suppress Rich broken-pipe/SystemExit failures after the pipe closes so shutdown can continue cleanly
• add coverage for forced terminal colors and broken-pipe handling

Testing

• uv run --group dev ruff check livekit-agents/livekit/agents/cli/cli.py tests/test_cli_log_level.py
• uv run --group dev pytest tests/test_cli_log_level.py -q

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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