Remove local generator code

[nogates]

What does this PR do?

Mirrors DataDog/datadog-api-client-go#4584 for the Python 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
• resolves scenario API, operation, parameter, and model attribute names from generated client metadata instead of depending on generator naming files

Testing

• PYTHONPATH=src DD_USE_GENERATED_TESTS=true python -m pytest tests -q --disable-warnings --maxfail=1 (1741 passed, 3166 skipped)
• selected cassette replay scenarios for SLOs, API keys, AuthN mappings, and IdP metadata (4 passed)
• core client, serialization, and deserialization tests (31 passed)
• 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

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 to include tests for that new functionality.