fix(codegen): preserve regenerated response aliases

[bokelley]

Supersedes #824 using an in-repository branch so required CodeQL and Argus review checks can run with normal repository permissions.

Includes the original generated-alias fix from #824 plus follow-up fixes:

• pins semantic response aliases with a regression test
• keeps generated Product aliases mypy-clean
• wraps alias fallback definitions for ruff

Validation run locally:

• uv run --extra dev ruff check src/
• uv run --extra dev mypy src/adcp
• uv run --extra dev pytest tests/test_code_generation.py -q --tb=short

[aao-ipr-bot]

LGTM. Follow-ups noted below. Replaces the eager getattr(_g, "FooResponse1", _g.FooResponse) fallback with a lazy try/except helper — the right shape now that regenerated _generated.py carries concrete *Response1 arms instead of *Response1 = *Response shims.

Things I checked

• Lazy fallback semantics (src/adcp/types/aliases.py:106-110): _generated_alias evaluates the fallback only on AttributeError. Old form forced eager evaluation of _g.FooResponse regardless. New form is import-time safe even when the parent union goes away upstream.
• Regression pin (tests/test_code_generation.py:32-44): regex getattr\(_g,\s*"[^"]+",\s*_g\.[A-Za-z_]+\s*\) matches only the attribute-fallback form, not the new string-fallback _generated_alias calls. Sound guard.
• Semantic alias resolution (tests/test_code_generation.py:54-104): pins ActivateSignalSuccessResponse → ActivateSignalResponse1, AcquireRightsAcquiredResponse → AcquireRightsResponse1, CreateMediaBuySubmittedResponse → CreateMediaBuyResponse3, plus six more. This is the actual fix — the original bug was these collapsing silently to the parent union.
• Wire-shape orderings (ad-tech-protocol-expert verified against upstream schemas/source/): AcquireRights oneOf is [Acquired, PendingApproval, Rejected, Error], matching aliases.py:642-652 (1→4). CreateMediaBuy [Success, Error, Submitted] matches aliases.py:461-479. UpdateMediaBuy same ordering matches aliases.py:607-614.
• ClaimType relocation (_generated.py:202-209): upstream verify-brand-claim-request.json defines claim_type as the singular task's discriminator, so the new import location is spec-correct. No external consumers of the old location in src/, tests/, or examples/.
• extract_exports_from_module AnnAssign extension (scripts/consolidate_exports.py:38-46): correctly picks up Foo: TypeAlias = Bar declarations the post-generate fixer emits. Unit test at tests/test_code_generation.py:46-52 covers it.
• _sync_protocol_envelope_import (scripts/post_generate_fixes.py:58-74): consolidates two prior call sites in restore_response_variant_aliases. Insertion is gated on actual ProtocolEnvelope references in the source, so the import doesn't get added speculatively. Regenerated acquire_rights_response.py:5-9 and create_media_buy_response.py:5-9 confirm the placement.

Follow-ups (non-blocking — file as issues)

• UpdateMediaBuyResponse1/2/3 coercion parity. The PR adds _ergonomic.py patches for CreateMediaBuyResponse1.packages and media_buy_status. The three update_media_buy arms have the same two fields plus affected_packages: Sequence[Package] and currently get no ergonomic coercion. Per code-reviewer this is a pre-existing gap, not a regression — but the same shape, same call ergonomics, should land together.
• Type erasure through _generated_alias (src/adcp/types/aliases.py:106). Returning Any means ActivateSignalSuccessResponse: TypeAlias = ActivateSignalResponse1 is Any to mypy on the public surface. The file's # mypy: disable-error-code="valid-type" header masks the diagnostic but doesn't restore concrete typing. Consider an if TYPE_CHECKING: branch that binds the alias to the known fallback class statically while keeping runtime lazy.
• Duplicate ClaimType in verify_brand_claims_response.py:21. Per ad-tech-protocol-expert: pydantic generates a separately-scoped ClaimType enum inside the plural response module. Adopters doing isinstance(result.claim_type, ClaimType) against the public re-export from the singular request module will silently fail (same string values, different class objects). Codegen-level dedup, not in scope here.
• Forward-compat on VerifyBrandClaimResponse (generated_poc/brand/verify_brand_claim_response.py:59). The Success | Error union has no Discriminator() callable and no UnknownStatus fallback arm. A new upstream oneOf arm will hit ValidationError on adopter deserialization. Same caveat applies to CreateMediaBuyResponse1.media_buy_status — coerce_to_enum returns the raw string on miss, which strict pydantic rejects.

Minor nits (non-blocking)

1. _generated_alias docstring (src/adcp/types/aliases.py:106-110). The helper has no docstring explaining why the helper exists or that fallback_name is the parent-union case. Two lines lifted from the PR body would prevent future churn.
2. Branch (c) in _sync_protocol_envelope_import (scripts/post_generate_fixes.py:74). The raw-prepend path fires only when neither from __future__ import annotations nor AdcpVersionEnvelope exists in the source — which python-expert confirmed never happens on generated_poc/ files. Defensive but dead. Either drop or add a # pragma: no cover hint.

Approving on the strength of the four new regression tests plus the upstream-schema oneOf verification.