PyYAML crash fixed; IAST abort and LLM ingestion bugs resolved

Bug Fixes

• bootstrap: keep yaml/_yaml loaded during module cleanup, fixing an issue that broke PyYAML consumers such as Airflow.

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• Code Security (IAST): fixes an issue where taint tracking could abort the Python process when old-style % string formatting handled tainted text containing literal IAST evidence marker delimiters.

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• Code Security (IAST): Resolve a weak-hash false positive reported on ddtrace's own code when IAST and the live debugger (Symbol Database) are both enabled.

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• LLM Observability: This fix resolves an issue where annotating a span with a metric key containing a dot (e.g. anomaly.query_count) prevented the span from being ingested. Dots in metric keys are now replaced with underscores.

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• LLM Observability: This fixes an issue where cache write input tokens were not captured by the OpenAI integration when using newer models that include cache write token charges.