fix: correct log level and remove redundant warning prefixes

[Ricardo-M-L]

Summary

• Change logger.error to logger.warning for cleanup errors in trace_batch_manager (cleanup is best-effort, and the message was prefixed with "Warning:" contradicting error level)
• Remove redundant Warning: prefix from warnings.warn() call in testing.py — warnings.warn already renders as UserWarning: ..., producing double-warning output

Test plan

• Verify log output is clean and consistent

🤖 Generated with Claude Code

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Note

Low Risk
Low risk: only adjusts log/warning messaging (no behavior or data flow changes), aside from reduced severity of a cleanup failure log line.

Overview
Tones down trace batch cleanup failure reporting by logging _cleanup_batch_data exceptions as warning instead of error.

Cleans up evaluation output by removing the redundant "Warning:" prefix from assert_experiment_no_regression’s warnings.warn message so warnings aren’t double-prefixed.

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