test(platform): isolate reverse cross-runtime acceptance

[XnLemon]

Scope

Extract Slice C from the former combined PR #62. This PR contains only the canonical reverse Runtime B -> A2A Router -> Runtime A acceptance, its deterministic Runtime B caller configuration, Runtime A responder capability, Router credential wiring, Compose/CI backend configuration, and the Spec 027 artifacts required to describe that scope.

It does not contain apps/console, frontend package import, browser acceptance, or the Slice D production Console integration.

Evidence

• NeKiro Issue [Spec 027] Prove canonical Runtime B to Router to Runtime A lineage #60 scope review: T024, recorded in Spec 027 tasks.
• Independent Slice C review: PASS, 0 High / 0 Medium; two non-blocking Low observations remain outside this scope because managed Router limits already cover the active request deadline and byte limits.
• Existing combined-PR CI run 30321032406 passed all checks for this same commit range.
• The fresh CI for this independent branch is required before merge.
• Local git diff check passed; local Go checks were not runnable because the workstation could not download uncached modules.

No new public contract, retry, fallback, alternate endpoint, direct Agent path, or runtime framework is introduced.

Closes #60

[codecov]

Codecov Report

❌ Patch coverage is 67.50000% with 65 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
agents/runtime-b/config.go	76.69%	12 Missing and 12 partials ⚠️
agents/runtime-b/nested.go	70.58%	10 Missing and 10 partials ⚠️
agents/runtime-b/handler.go	33.33%	12 Missing and 2 partials ⚠️
agents/runtime-b/cmd/runtime-b/main.go	0.00%	7 Missing ⚠️

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