PyYAML cleanup fixed; IAST false positives 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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• IAST: This fix resolves an issue where IAST could report a false positive vulnerability against a request whose input did not actually contain tainted data. A concurrent or still-open request holding a tainted value could leak its taint into the current request's checks. Queries are now scoped to the calling request's slot.

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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.