feat(v3-ref-seller): enable server-side ValidationHookConfig at boot

[bokelley]

Context

The v3 reference seller does not pass validation=ValidationHookConfig(requests="strict", responses="strict") to serve(), so the SDK's request/response validators are not run. Bugs like the recent pricing_options shape conformance issue would have been caught immediately had validation been on.

Refs:

• examples/v3_reference_seller/src/app.py (where serve() is invoked)
• src/adcp/server/serve.py (ValidationHookConfig parameter)

Acceptance Criteria

• V3 reference seller passes validation=ValidationHookConfig(requests="strict", responses="strict") to serve() by default
• README documents the trade-offs: strict mode rejects out-of-spec requests/responses at the boundary; "warn" is the relaxed alternative
• An env var or config knob lets adopters drop to "warn" if they need to tolerate older buyers/sellers during a rollout
• Smoke test: an intentionally malformed request is rejected with a spec-shaped error response in strict mode
• Smoke test: same malformed request produces a warning (logged, but processed) in warn mode

[bokelley]

Triage

Classification: Feature request
Bucket(s): examples, validation, client
Status: drafting-pr
Milestone: omit

What the experts said:

• dx-expert: Threading validation through serve() → create_mcp_server() → _register_handler_tools() → create_tool_caller() is the right seam; test at create_tool_caller level to stay DB-free; response-strict errors lack field pointers (friction but not a blocker).
• code-reviewer: Non-breaking confirmed; A2A silent-drop is a blocker — transport="both" leaves A2A dispatch unvalidated unless ADCPAgentExecutor also receives validation=; env-var should reuse ADCP_ENV convention to stay consistent with _default_response_mode() rather than introducing a parallel ADCP_VALIDATION_MODE knob.

My take: The SDK machinery exists (create_tool_caller already accepts ValidationHookConfig) — this is purely a threading gap from serve() down to both the MCP and A2A executors, plus example + doc work. Drafting a PR that threads validation= through serve(), create_mcp_server(), create_a2a_server(), and the A2A executor; the reference seller reads ADCP_ENV (prod → warn, otherwise strict) to stay aligned with the client-side default.

PR summary: Thread validation: ValidationHookConfig | None = None through adcp.server.serve(), create_mcp_server(), create_a2a_server(), and ADCPAgentExecutor; update the v3 reference seller to boot with strict validation defaulting to warn when ADCP_ENV=production; add README docs and smoke tests.

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Triaged by Claude Code. Session: https://claude.ai/code/cse_01L78kF12Kbmf1QvBCuN5k5W

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