Streaming hardening: cancellation leak + concurrent-composition name collision (#4499, #4500)

[Skobeltsyn]

Streaming hardening — two real defects found via TDD probes, fixed

Follow-up to #199: probing complex-composition streaming for collisions and leaks surfaced two genuine bugs. RED probe tests written first, then fixed.

#4499 — cancelling a session no longer leaks the invocation

AgentSession documented that cancelling events collection cancels the underlying invocation. It didn't: the producer ran in a detached SupervisorJob + Dispatchers.Unconfined scope the collector couldn't reach, so a cancelled — or take(1)-abandoned — consumer left the agent making model calls in the background (runaway-work / resource leak + broken contract).

Root cause confirmed by instrumentation: a downstream onCompletion stage is skipped when the collecting coroutine is cancelled externally. Fix puts the scope teardown in the events flow's own flow {} finally, which fires on normal completion, take(1), AND external cancellation; await() cancels the same scope on its own cancellation. Bare-cancellation contract (no synthetic Failed) preserved. Both session producers covered.

#4500 — concurrent composition rejects duplicate agent names

Parallel (/) and Forum (*) demux streamed events by agentId (the agent's name). Two participants sharing a name produced indistinguishable interleaved streams. The single-placement rule caught the same instance placed twice but not two distinct same-named instances — now construction fails loud with a message naming the duplicate, the same stance as duplicate tool/skill names. speculative(n) self-racing is the documented exception.

Tests (TDD, fail-first)

• SessionCancellationLeakProbeTest (3) — collector-cancel mid-stream, take(1), and await()-cancel each tear down a parked chatStream invocation.
• CompositionNameCollisionProbeTest (4) — duplicate names rejected in parallel/chained-parallel/forum; distinct names accepted.
• ComplexStreamingTest (3) — deep chain (head → 3-way parallel → reducer), forum stage, loop stage: no collision (distinct agentIds), no loss (droppedEvents == 0), single terminal.

Gates: full ./gradlew test (all modules, TEST-*.xml clean) + ./gradlew build -x test (detekt) — both green. Docs: CHANGELOG, streaming.md (cancellation contract rewritten honestly + known-gap closures), composition.md, internals adjunct.

Stacked on feat/antifragility-pass (#199) since both touch the session files it modifies; retargets to main once #199 lands. CodeQL java-kotlin expected-red per #4383.

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