[codex] Add attachment update 4999 reproducer

[Flerpharos]

Summary

• add an explicitly gated live integration reproducer for attachment update error 4999
• report the update endpoint, object identifiers, request parameters, payload size, and payload hash
• document Bash and PowerShell commands for maintainers to rerun the reproducer

Why

The existing integration suite performs attachment updates only as part of a broader metadata scenario. This focused path creates a minimal isolated attachment, captures the request shape, and reports a structured response when LabArchives returns 4999.

Validation

• uv run --python 3.10 pytest --no-cov --integration tests/test_integration.py -k attachment_update_4999_reproducer (skipped as expected without the opt-in environment variable)
• uv run --python 3.10 pre-commit run --all-files
• uv run --python 3.10 pre-commit run --all-files --hook-stage pre-push
• uv run --python 3.10 sphinx-build -b html docs/source docs/_build/html

The live LabArchives update was not run because the explicit reproducer gate was left disabled.

Fixes #155