feat(server/a2a): structured error parity with MCP — emit field/details/retry_after, catch decisioning AdcpError

[bokelley]

Summary

Closes #530. Mirrors the MCP-side fix from #525 onto the A2A surface so storyboards graded against A2A-served sellers see the same structured adcp_error envelope as MCP buyers. Without this, A2A surfaces would surface a wire-conformance gap once the storyboard runner's structured-error checks run against A2A (or via the upstream storyboard runner UX work in adcp-client#1527).

Two parity gaps closed

1. Catch both exception hierarchies. ADCPAgentExecutor.execute() now catches both adcp.exceptions.ADCPError (client-side framework error) AND adcp.decisioning.types.AdcpError (server-side structured error platform methods raise). They are disjoint hierarchies — pre-fix the decisioning case fell into except Exception and rendered as plain "Skill execution failed" text, losing the structured shape entirely for adopters using DecisioningPlatform.

2. Emit the full envelope. _send_adcp_error now projects code, message, recovery, field, suggestion, retry_after, details — populated when the raised exception supplies them, omitted when None. Pre-fix only code / message / recovery / suggestion reached the wire.

Reuse, not duplication

Field extraction is shared with the MCP path via adcp.server.translate._extract_structured_fields (the helper #525 factored out for exactly this purpose). Both transports project off the same source-of-truth shape — no drift between MCP and A2A error envelopes.

_extract_structured_fields was reusable as-is — no factoring needed.

Test plan

• Unit tests parametrized over the same code set as feat(server): MCP error responses populate structuredContent.adcp_error (closes #509) #525's MCP test (MEDIA_BUY_NOT_FOUND, PACKAGE_NOT_FOUND, TERMS_REJECTED, BUDGET_TOO_LOW)
• Decisioning AdcpError reaches A2A DataPart as structured envelope (the main parity gap — was dropped pre-fix)
• All optional fields (field, details, retry_after, suggestion) populate when present, omit when absent
• ADCPTaskError (the only path that worked pre-fix) regression: still caught, still projects structured envelope, field populates when carried on Error.field
• Existing tests/test_a2a_server.py suite stays green (40 tests)
• Full repo regression: 3795 passed
• ruff check + mypy src/adcp/server/ clean

Refs

• feat(server/a2a): structured error parity with MCP — emit field/details/retry_after, catch decisioning AdcpError #530 — this issue
• feat(server): MCP error responses populate structuredContent.adcp_error (closes #509) #525 — sibling MCP-side fix (closed feat(server): MCP error responses must populate structuredContent.adcp_error.code (not just text) #509). Reused _extract_structured_fields directly
• adcp-client#1527 — upstream storyboard runner UX. When the runner adds structured-error checks to A2A storyboards, this gap would have become a wire-conformance failure

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