feat(webhooks)!: replace domain kwarg with typed protocol (AdcpProtocol enum)

[bokelley]

Follow-up to #632. The schema's mcp-webhook-payload.json defines a typed protocol field (AdcpProtocol enum: media-buy | signals | governance | creative | brand | sponsored-intelligence); the builder was instead accepting an undocumented domain string and stuffing it into the wire body via extra='allow'. The JS reference implementation (buildTaskWebhookPayload in from-platform.ts) sets payload.protocol and never sets domain — so the Python SDK was the outlier.

Summary

• create_mcp_webhook_payload(..., domain=...) → create_mcp_webhook_payload(..., protocol=...)
• WebhookSender.send_mcp(..., domain=...) → WebhookSender.send_mcp(..., protocol=...)
• protocol accepts AdcpProtocol | str | None; unknown values raise ValidationError at construction.
• AdcpProtocol is now publicly exported from adcp.types.

Why a clean break (no domain deprecation alias)

• domain was never in the spec; no schema contract to honor.
• Zero in-repo callers passed domain= to the builder (verified by grep).
• Co-ships with feat(webhooks)!: create_mcp_webhook_payload returns McpWebhookPayload #632's migration cycle — adopters touch the call sites once, not twice.

Cross-SDK alignment

Verified in adcontextprotocol/adcp-client (the JS SDK):

• buildTaskWebhookPayload sets payload.protocol = protocolForTool(opts.tool) (same AdcpProtocol 6-value enum).
• The closed 20-value TaskType is enforced as SPEC_WEBHOOK_TASK_TYPES at the emitTaskWebhook dispatch layer (skip-with-warning vs. our raise-at-construction — both spec-conformant; different surfaces).

Follow-up worth filing

The JS SDK auto-derives protocol from the tool name (protocolForTool(opts.tool)). Our Python builder leaves protocol as a manual kwarg. A future enhancement could mirror the JS behavior — caller passes only task_type, builder fills in protocol from a TaskType → AdcpProtocol mapping. Out of scope here; flagging.

BREAKING CHANGE: domain kwarg removed. Migrate to protocol.

Test plan

• pytest tests/ — 4211 passed (added 1 new test for the protocol kwarg accepting enum/string and rejecting unknowns)
• mypy src/adcp/ — clean
• ruff + black — clean
• public_api_snapshot.json regenerated to include AdcpProtocol
• Cross-SDK enum source verified (Python's TaskType is auto-generated from schemas/cache/enums/task-type.json; same source the JS SPEC_WEBHOOK_TASK_TYPES derives from)

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