Remove local generator code - #4327

What does this PR do?

Mirrors DataDog/datadog-api-client-go#4584 for the Java client:

• removes the in-repository Python generator package, templates, tests, and generation script while retaining the OpenAPI snapshots under .generator/schemas
• removes generator-only pre-commit hooks
• documents the internal client-generation and pull-request regeneration workflow

Testing

• DD_USE_GENERATED_TESTS=true mvn test (1,966 tests, 37 skipped)
• pre-commit validate-config
• pre-commit run docs --hook-stage manual --files README.md --show-diff-on-failure
• pre-commit run check-case-conflict --all-files --show-diff-on-failure
• git diff --check

Additional Notes

The plain cassette replay suite currently contains pre-existing response expectation drift and missing freeze files. The generated-test mode used by CI passes.

Review checklist

• This PR includes all newly recorded cassettes for any modified tests. (Not applicable; no tests or cassettes changed.)

• This PR does not rely on API client schema changes.

  • The CI should be fully passing.

• Or, this PR relies on API schema changes and this is a Draft PR that includes tests.

  • Note: CI should not be run on this Draft PR, as it is expected to fail without the corresponding schema changes.