test(server): regression test for context-echo on dispatcher-wrapped non-AdcpError

[bokelley]

Closes #562 (verification follow-up to #560).

Why

PR #560 added context-echo on the AdcpError raise path. The follow-up question was: does the dispatcher's exception-wrap path preserve that echo when a non-AdcpError exception escapes a platform method?

What I verified

`_invoke_platform_method` (`src/adcp/decisioning/dispatch.py`) catches every non-AdcpError in its `except Exception` clause and wraps to `AdcpError("INTERNAL_ERROR", recovery="terminal")`. Once wrapped, the AdcpError travels up to `serve.py`'s decisioning branch, which builds the structured envelope via `build_mcp_error_result(exc, params=kwargs)` — so context is echoed onto the wire.

This test pins the chain end-to-end:

1. `ValueError("oops")` raised inside the platform method
2. Wrapped to `AdcpError("INTERNAL_ERROR")` by `_invoke_platform_method`
3. Caught by `serve.py:1798`'s decisioning branch
4. Projected via `build_mcp_error_result` with `params=kwargs`
5. Wire result carries `structuredContent.adcp_error.code = "INTERNAL_ERROR"` AND `structuredContent.context = {"correlation_id": "buyer-req-562"}`

If a future refactor lets a non-AdcpError exception escape the dispatch wrap (early-return path, middleware short-circuit, asgi_middleware that catches and re-raises differently), this test fails first — adopters lose correlation IDs across the boundary and we know about it.

Scope

Test-only change, no production code modified. The DecisioningPlatform path is fully covered. Raw `ADCPHandler` users (no DecisioningPlatform) currently see FastMCP's generic error on un-wrapped exceptions — separate concern; not addressed here.

Test plan

• `pytest tests/test_mcp_structured_error.py::test_non_adcp_error_via_dispatch_wrap_still_echoes_context` — pass
• All existing structured-error tests — no regressions
• `ruff check` — clean

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