feat(testing): forward pre_validation_hooks through build_asgi_app

[bokelley]

Closes #651

Summary

build_asgi_app and build_test_client were missing pre_validation_hooks from their signatures. Because create_mcp_server already accepts this parameter, in-process ASGI tests silently got a different validation surface than production serve() calls — pre-v3 buyer payloads that serve(pre_validation_hooks=...) would coerce were rejected by build_asgi_app-driven tests, causing false-red test failures and false-green production gaps.

This PR adds pre_validation_hooks: dict[str, Callable[..., Any]] | None = None to both helpers and forwards it to the existing create_mcp_server call. No change to internals — pure forwarding, fully backwards-compatible.

What was tested

• pytest tests/test_testing_decisioning.py tests/test_pre_validation_hooks.py → 41 passed
• pytest tests/ -k "not test_real_tls" --ignore=tests/integration → 4318 passed (TLS test is a pre-existing network-dependent failure unrelated to this change)
• ruff check src/adcp/testing/decisioning.py → all checks passed
• mypy src/adcp/testing/ → no new errors

Pre-PR review

• code-reviewer: approved — no blockers; 1 nit fixed (dropped dead hook_calls variable from smoke test)
• dx-expert: approved — no blockers; 1 nit fixed (same)

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_01H8Trwexkgv1VyFJwDdZMoa

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