fix(types): add CreateMediaBuySubmittedResponse alias and fix handler return type

[bokelley]

Closes #570

Summary

Three non-breaking SDK fixes for the async media-buy submitted branch, surfaced by the FastMCP validator misreport. Root cause (confirmed empirically): when a status='submitted' response is missing task_id, all three anyOf branches fail validation and jsonschema surfaces the deepest-path error — a misleading 'submitted' is not one of [...] enum mismatch on Branch 0 — rather than the actual required: task_id failure on Branch 2.

Changes:

1. src/adcp/decisioning/handler.py — Fix PlatformHandler.create_media_buy return annotation from CreateMediaBuySuccessResponse to CreateMediaBuyResponse (the three-branch union). The previous annotation was a static lie that misled mypy users and code reviewers.
2. src/adcp/types/aliases.py / __init__.py / adcp/__init__.py — Add CreateMediaBuySubmittedResponse = CreateMediaBuyResponse3 alias and export it from all three public surfaces so implementors have a semantic name instead of the internal Response3 suffix.
3. schemas/cache/media-buy/create-media-buy-async-response-submitted.json — Add status (const "submitted") and task_id (string, required) to the standalone submitted schema. With response_mode="strict" the SDK's own validate_response() will now catch a missing task_id with a clear message.
4. src/adcp/types/guards.py — Update CreateMediaBuyResponse local union alias to include the submitted branch; fix is_create_media_buy_success() to return False for submitted envelopes; add is_create_media_buy_submitted() TypeGuard.
5. src/adcp/types/generated_poc/media_buy/create_media_buy_async_response_submitted.py — Bring the standalone generated class in sync with the updated schema (add status + task_id required fields).
6. tests/fixtures/public_api_snapshot.json — Regenerated to include new exports.

Nit (not fixed, noted for reviewers): Four sibling standalone async-submitted schemas (update-media-buy, build-creative, get-products, sync-catalogs) remain underspecified in the same way. Canonical definitions in the bundled response schemas enforce required: [status, task_id] correctly; follow-up to bring standalone schemas to parity.

What tested

• ruff check — passes on all modified files
• mypy on modified files — no new errors (pre-existing errors in unrelated files unchanged)
• pytest -k "media_buy or create_media or public_api" — 135 passed, 12 skipped

Pre-PR review

• code-reviewer: approved after fixup iteration — blockers (snapshot stale, guards missing submitted branch, generated file not updated, top-level package missing export) all addressed; 1 nit noted (sibling schemas underspecified, out of scope for this PR)
• ad-tech-protocol-expert: approved — task_id is unambiguously required per AdCP async task contract (confirmed via create-media-buy-response.json Branch 2 and comply-test-controller-request.json); additionalProperties: true correct and consistent with established pattern; change is non-breaking per spec

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See adcp#3121
for context.

Session: https://claude.ai/code/session_01MXKE7txstMT3B7aTctfXcZ

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[bokelley]

Superseded by #575 (merged today as f7960ed). The handler return-type widening + CreateMediaBuySubmittedResponse alias landed via that PR; closing as duplicate scope.

[bokelley]

Acknowledged — confirmed #575 (f7960ed) covers the handler return-type widening and CreateMediaBuySubmittedResponse alias. No further action needed here.

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