Align request-validation error codes with AdCP storyboard semantics

[jstar0]

Align request-validation error codes with AdCP storyboard semantics

Problem

Request-schema failures are currently normalized to VALIDATION_ERROR at the MCP TypeAdapter boundary. AdCP 3.1.0-beta.3 grades structural request failures as INVALID_REQUEST, including BR-UC-002-create-media-buy.feature scenario @T-UC-002-inv-015-6 for a package missing product_id.

PR #1534 preserves the current error code while adding a strict xfail tripwire for this mismatch. A separate change is needed because the classification must remain consistent across MCP, A2A, and REST rather than special-casing one test or transport.

Expected behavior

• Structural failures such as missing, malformed, or unknown request fields emit INVALID_REQUEST.
• Value-constraint failures on structurally valid fields emit VALIDATION_ERROR.
• MCP, A2A, and REST expose the same classification for equivalent failures.
• Buyer-visible messages remain sanitized and do not include raw Pydantic input values or documentation URLs.

Acceptance criteria

• Add table-driven coverage for representative structural and value-constraint failures across the relevant transport boundaries.
• Implement a shared classification rule instead of transport-specific branches.
• Remove the strict xfail added in PR fix: wrap MCP TypeAdapter validation errors #1534 by making its storyboard case pass.
• Preserve the existing two-layer AdCP error envelope and structured field/details metadata.

[numarasSigmaSoftware]

Spec-adjudication note for the classification decision here (and the #1694 regeneration): the pinned 3.1.1 enums/error-code.json descriptions place "violates schema constraints" under INVALID_REQUEST and reserve VALIDATION_ERROR for failures "beyond schema validation" — so regenerating the UC-003 revision outlines to grade VALIDATION_ERROR would move the storyboard away from the enum's own taxonomy, while the current storyboard grade (INVALID_REQUEST) matches it better. Production currently emits VALIDATION_ERROR for schema-invalid revision via the shared validation boundary (pinned per transport in tests/integration/test_update_media_buy_revision_validation_wire.py). Both codes are correctable, so buyer recovery is unaffected either way — but the reconciliation direction should be decided from the enum text (and any error-handling prose), not anchored on current production behavior.