feat(testing): add SellerTestClient for in-process handler testing

[bokelley]

Refs #662

Adds SellerTestClient — an in-process MCP test harness that eliminates the per-file boilerplate of constructing JSON-RPC envelopes, calling mcp.call_tool(), and extracting adcp_error from structuredContent. Adopters now get a single typed call that handles all three.

from adcp.testing import SellerTestClient

@pytest.fixture
def seller():
    return SellerTestClient(MySeller())

async def test_buy_not_found(seller):
    result = await seller.invoke("update_media_buy", {"media_buy_id": "missing", ...})
    assert not result.passed
    assert result.adcp_error.code == "MEDIA_BUY_NOT_FOUND"

The harness calls mcp.call_tool() in-process — it exercises the full handler dispatch and error-translation stack without HTTP or SSE framing. For HTTP-level tests (auth, CORS, middleware), build_test_client remains the right tool.

What this PR does not include (see #662 for status):

• run_scenario() — requires bundled compliance scenario playbooks that are not yet in the SDK
• A2A transport — served via a separate ASGI app; needs its own in-process client primitive

What was tested

• pytest tests/test_seller_test_client.py — 18 new tests, all passing
• pytest tests/ -q --ignore=tests/integration — 4447 passed, 1 pre-existing network flake (test_real_tls_handshake_still_validates_hostname)
• ruff check src/adcp/testing/ — no issues
• mypy src/adcp/testing/harness.py --ignore-missing-imports — no new errors from this file

New public API

Symbol	Module	Notes
SellerTestClient	adcp.testing	In-process MCP harness; takes DecisioningPlatform
ToolInvokeResult	adcp.testing	Return type with .passed, .data, .adcp_error, .raw
AdcpErrorPayload	adcp.testing	Typed container for wire adcp_error fields

validation=None (default) disables schema validation so tests target handler behavior. Pass SERVER_DEFAULT_VALIDATION to match production behavior (build_test_client does this by default — see migration note in __init__ docstring).

Nits surfaced by review (not fixed in this PR)

• Private _mcp attribute accessed in two tests — low-value coupling to implementation detail; safe to remove in a follow-up cleanup
• docs/testing-guide.md has no mention of SellerTestClient — should be added once the API stabilises
• result.data may include context echo keys alongside business-payload keys when context-echo is active — documented in raw as the escape hatch

Pre-PR review

• code-reviewer: approved — fixed both flagged issues (asyncio.Lock on _ensure_mcp; RuntimeError fallthrough guard for unknown FastMCP return shape)
• dx-expert: approved — fixed DX blocker (removed unused # type: ignore[misc]); nits noted above

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_01VZRaadSyoHDm7CU945e3Ba

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Generated by Claude Code

[bokelley]

Fold candidate from #678. Issue #678 proposes SellerA2AClient — a sibling in-process test harness for A2A adopters. It's same-author, same adcp.testing scope, and explicitly defers to this PR as the reference point.

Before this PR merges, consider whether to stub out SellerA2AClient (even just the class skeleton + __all__ entry) so adopters have the surface to import from. A minimal scaffold here would prevent a cold restart in #678 and preserve the design context while the SellerTestClient API is still fresh.

Suggestion only — skip if scope feels right as-is.

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Triaged by Claude Code. Session: https://claude.ai/code/session_015nGuiswmggSyTdEinF6SgX

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Generated by Claude Code