testing: ship adcp.testing.make_request_context + build_asgi_app helpers

[bokelley]

From the salesagent v3.12 → 4.x migration (SDK_FEEDBACK.md, items #10 and #11).

Two test-DX gaps in adcp.testing/:

1. make_request_context

RequestContext has 11 fields, 4 with <factory> defaults. For unit testing through PlatformHandler, the only thing the handler reads is ctx.account and sometimes ctx.request_id.

Constructing a RequestContext for tests requires guessing which factory defaults are safe. I hit TypeError: unexpected keyword argument 'adopter_request' when I read a docstring that mentioned the field but it's not a constructor parameter. The test-friendly path took several tries.

Ask: ship adcp.testing.make_request_context(account=..., **overrides) with sane defaults and a stable signature documented as the test seam.

2. build_asgi_app

create_adcp_server_from_platform doesn't accept name=, but serve() does.

Building the ASGI app in tests requires going through create_adcp_server_from_platform() → create_mcp_server() and the kwarg surface differs between the two. The current docstring on create_mcp_server is a great implementation reference but it's not framed as the test seam.

Ask: first-class adcp.testing.build_asgi_app(platform, name=...) → ASGIApp helper that's officially the "for tests" way to get a running server.

Acceptance

Adopters writing platform-handler unit tests can:

1. Construct a RequestContext in one line with no factory-default guessing.
2. Get an ASGIApp from a platform in one call with the same kwarg surface as serve().

Both helpers live in adcp.testing/ (module already exists with test_helpers.py).

[bokelley]

Triage

Classification: Execute PR — clear spec, self-contained module, no protocol changes, no schema impact.

Scope: Two new helpers in adcp.testing:

1. make_request_context(account, account_id, **overrides) — constructs a RequestContext with all framework-owned stubs pre-wired. caller_identity defaults to account.id for test convenience (production uses a composite opaque key — tests should check is not None, not the exact value).
2. build_asgi_app(platform, *, name=None) — returns a runnable ASGI callable with test-appropriate defaults (auto_emit_completion_webhooks=False, thread_pool_size=1, enable_dns_rebinding_protection=False). Synchronous — must be called outside a running event loop.

Key design decisions:

• caller_identity is defaulted to account.id (not the production composite key <store>:<account_id>) so tests don't need to know the store internals. Docstring warns adopters not to assert the exact value.
• build_asgi_app sets auto_emit_completion_webhooks=False to bypass the F12 webhook-sender gate, and enable_dns_rebinding_protection=False to allow httpx.ASGITransport test clients.
• Both helpers use lazy imports to avoid pulling the decisioning module into processes that don't need it.

PR: #532 (draft)

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