feat(python): rename DD_TRACE_ATEXIT_ENABLED to DD_TRACER_ATEXIT_HOOKS in config normalization rules

[ZStriker19]

Summary

• Renames DD_TRACE_ATEXIT_ENABLED → DD_TRACER_ATEXIT_HOOKS in python_config_rules.json
• Follows review feedback on feat(tracing): add DD_TRACE_ATEXIT_ENABLED to skip shutdown hook registration dd-trace-py#17195 to use a more scoped name that makes clear this only affects the tracer and LLMObs (not all ddtrace products)

Replaces #6653 (which used the old name).

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