feat(python): add DD_TRACE_ATEXIT_ENABLED to config normalization rules

[ZStriker19]

Summary

• Adds DD_TRACE_ATEXIT_ENABLED to python_config_rules.json normalization rules
• Fixes test_config_telemetry_completeness failure caused by dd-trace-py/dd-trace-py#17195

DataDog/dd-trace-py#17195

Context

dd-trace-py#17195 adds DD_TRACE_ATEXIT_ENABLED (a Python-only config). Since _get_config from the telemetry module auto-registers every config it reads, the new key appears in telemetry payloads and needs a normalization rule here.

Normalized name: trace_atexit_enabled (follows existing DD_TRACE_* → trace_* pattern in this file).

[github-actions]

CODEOWNERS have been resolved as:

utils/telemetry/intake/static/python_config_rules.json                  @DataDog/apm-sdk

[datadog-prod-us1-3]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

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