fix(decisioning): wrap pydantic.ValidationError from delegates as INVALID_REQUEST

[bokelley]

Closes #652

Summary

Platform delegates that manually call RequestModel.model_validate() against a stricter subclass schema previously surfaced as INTERNAL_ERROR (terminal) when the buyer's input failed validation, stripping all field information from the wire envelope. This PR converts those pydantic.ValidationError exceptions to INVALID_REQUEST (correctable) with populated field and details.validation_errors, matching the behaviour already present in _coerce_params_to_platform_type for the annotation-coercion path.

Two changes:

1. _validation_error_to_invalid_request helper — builds the AdcpError("INVALID_REQUEST") with narrow_union_errors-filtered field paths, with a safe fallback if the narrowing import fails. Input values are excluded from the error payload on both paths (include_input=False) to avoid leaking buyer-supplied data.

2. except _ValidationError clause in _invoke_platform_method — inserted between except TypeError and except Exception, fires on_failure hook (so proposal-flow reservations are released) before re-raising the wrapped error.

Note: the framework cannot distinguish a buyer-request ValidationError from a seller-response construction ValidationError at this catch boundary. The tradeoff is intentional and consistent with _coerce_params_to_platform_type — surfacing INVALID_REQUEST correctable with field paths is strictly more informative than INTERNAL_ERROR terminal in either case.

What was tested

• pytest tests/test_decisioning_dispatch.py → 56 passed (includes updated test_invoke_validation_error_surfaces_narrowed_field_paths and new test_invoke_validation_error_from_manual_model_validate_is_invalid_request)
• pytest tests/ --ignore=tests/integration → 4298 passed, 0 new failures (1 pre-existing TLS integration test excluded)
• ruff check src/adcp/decisioning/dispatch.py → all checks passed
• mypy src/adcp/decisioning/dispatch.py --ignore-missing-imports → no new errors on changed lines

Pre-PR review

• code-reviewer: approved after 1 blocker fixed — fallback exc.errors() call now passes include_input=False, include_context=False to prevent buyer-supplied data leaking into details.validation_errors; 1 nit noted (exc: Any signature loses static type info)
• dx-expert: approved — field dot-join produces correct "extra_field" for extra='forbid' errors; details shape asymmetry from _internal_error_details is intentional; no suggestion field is consistent with all peer INVALID_REQUEST wraps in the same file

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_01J7nYYToyy7PCs6CgQGRAWN

---

Generated by Claude Code