feat(client): surface structured adcp_error on TaskResult

[bokelley]

Summary

Closes the SDK's last politeness move on the buyer error path: TaskResult.adcp_error is now a first-class field carrying the seller's full structured error envelope, independent of debug mode.

Previously the SDK paraphrased a seller's structured adcp_error (code, field, details, recovery, ...) into a flat error: str and gated the structured form behind debug=True. That made spec-shaped errors invisible to alerting, version-compat probes, and grader assertions in production.

This is the Python analog of the JS-side issue 1679 ("flatten wire INVALID_REQUEST detail to {}") — same disease, slightly different shape: Python doesn't flatten to {}, it flattens to a string and hides the structure behind a debug flag.

What changes

• TaskResult.adcp_error: dict[str, Any] | None — new public field, populated whenever the wire carried a spec-shaped envelope.
• MCP: FAILED-path TaskResult carries the already-validated adcp_error dict (was previously only in debug_info.response).
• A2A: FAILED-path now extracts the adcp_error DataPart per transport-errors.mdx §A2A Binding. When the seller omits the TextPart, the structured envelope's message/code synthesizes the error string so adopters don't see the "Task failed" placeholder mask a real diagnostic.
• Shared validator: protocols/_adcp_errors.py holds the spec-defined size/shape caps (code non-empty string ≤ 64 chars, total ≤ 4 KB), used by both transports.

Adopter ergonomics

result = await client.create_media_buy(...)
if result.adcp_error and result.adcp_error["code"] == "INVALID_REQUEST":
    field = result.adcp_error.get("field")
    ...

The flat error: str stays for back-compat (human messages, logging).

Tests

• MCP: structuredContent.adcp_error and text-fallback both surface on TaskResult.adcp_error with debug=False.
• A2A: failed task with adcp_error DataPart surfaces structurally; envelope-only (no TextPart) synthesizes the human message.
• Existing "no structured envelope" tests gained an assertion that adcp_error stays None.

Test plan

• pytest tests/test_protocols.py tests/test_mcp_structured_error.py tests/test_a2a_structured_error.py tests/test_mcp_extraction.py — 194 passed
• Broader sweep including idempotency, error-code-conformance, translate, capabilities — 361 passed
• ruff check clean on changed files
• mypy clean on changed files
• Public API snapshot tests still pass

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