fix(types): add UpdateMediaBuyResponse1 ergonomic coercion

[sangilish]

Summary

Follow-up to #854.

This extends the generated ergonomic coercion pass to include the update-media-buy success response arm. UpdateMediaBuySuccessResponse / UpdateMediaBuyResponse1 now gets the same package-subclass and media_buy_status string coercion treatment that CreateMediaBuyResponse1 already had.

What changed

• Added UpdateMediaBuyResponse1 to scripts/generate_ergonomic_coercion.py response analysis.
• Regenerated src/adcp/types/_ergonomic.py so affected_packages, packages, and media_buy_status are patched at import time.
• Added regression coverage for package subclass inputs on both update response package fields and string media_buy_status coercion.

Testing

• uv run --extra dev pytest tests/test_type_coercion.py tests/test_code_generation.py -q --tb=short
• uv run --extra dev pytest tests/test_type_aliases.py tests/test_public_api.py tests/type_checks -q --tb=short
• uv run --extra dev pytest -q --tb=short
• uv run --extra dev ruff check scripts/generate_ergonomic_coercion.py src/adcp/types/_ergonomic.py tests/test_type_coercion.py
• uv run --extra dev mypy src/adcp/types
• uv run --extra dev mypy src/adcp

GitHub CI: all normal CI jobs are green, including Python 3.10-3.13, Postgres conformance, schema validation, downstream import smoke, storyboards, IPR, and GitGuardian.

Note: full uv run --extra dev ruff check still reports unrelated existing lint failures in tests outside this PR's touched files, so I used the targeted ruff command above for the changed files.

CI note: the only failing GitHub check is code_review (Argus). It fails before review execution while minting the GitHub App token because the app client id / app id secret is unavailable in this fork PR context. This is not a code/test failure in this branch.

[bokelley]

Superseded by #864, which mirrors this follow-up onto a repo-owned branch so the required base-repo checks can run.