fix(webhooks): correct type annotations for extract_webhook_result_data and payload builders

[bokelley]

Closes #598

Summary

Two webhook helpers in src/adcp/webhooks.py declared type annotations that didn't match runtime behavior, forcing adopters to cast or add # type: ignore comments.

extract_webhook_result_data — declared return type was AdcpAsyncResponseData | None but every code path returns dict[str, Any] | None. The cast() calls inside the body were lies to mypy; callers that did result.get(...) correctly got a type error. Return type corrected to dict[str, Any] | None; docstring updated accordingly.

create_mcp_webhook_payload and create_a2a_webhook_payload — result parameter was typed as AdcpAsyncResponseData | dict[str, Any] (narrow discriminated union), but both functions already called hasattr(result, "model_dump") at runtime and accepted any Pydantic model. Parameter widened to PydanticBaseModel | dict[str, Any]; docstrings updated.

Also removes the direct import from adcp/types/generated_poc/ in webhooks.py (a CLAUDE.md layering violation — that import path is for stable.py / aliases.py / _ergonomic.py only). The import is now unused after the above fixes.

Fixes bogus message= kwarg in the create_a2a_webhook_payload docstring example, which would raise TypeError if copied verbatim.

Not in scope / noted for follow-up: WebhookSender.send_mcp in webhook_sender.py:525 has the same narrow AdcpAsyncResponseData | dict[str, Any] | None annotation and the same generated_poc/ import. That file is separate from the callsites named in #598 — addressed in a follow-up.

What tested

• pytest tests/test_webhook_handling.py tests/test_webhooks_deliver.py tests/conformance/signing/ — 164 passed, 15 skipped
• Added TestWebhookPayloadBuilderPydanticModel (4 tests) exercising the model_dump path in both builders via a plain BaseModel subclass — previously untested code path
• mypy src/adcp/webhooks.py — clean
• ruff check src/adcp/webhooks.py — clean

Pre-PR review

• code-reviewer: approved — all blockers resolved, no new issues (round 2)
• dx-expert: approved — all blockers resolved (round 2)

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[bokelley]

Fold opportunity from #601: Issue #601 proposes adding to_wire_dict — a public wrapper around the same Task | TaskStatusUpdateEvent | Pydantic | dict dispatch that _payload_to_dict (private) already implements in this file — plus an optional return-type change to create_mcp_webhook_payload that directly overlaps this PR's annotation work.

If you want to ship them together, consider folding before merge: expose _payload_to_dict as to_wire_dict, re-export it from adcp.webhooks and the package root, and add the two roundtrip tests from #601's acceptance criteria. The implementation is small given _payload_to_dict already exists. If you prefer to keep this PR scoped to annotations, #601 will resurface on merge automatically.

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