Resolve #2734: Reject managed SSE without response stream support

[ayden94]

Summary

Managed SSE async iterables were silently reported as handled when the active adapter exposed no FrameworkResponse.stream, swallowing the requested event stream without an observable error. This aligns managed SSE with the documented adapter contract by surfacing a clear unsupported-stream failure before marking the response handled.

Linked context: Closes #2734

Changes

• packages/http/src/dispatch/dispatcher.ts: writeManagedSseIterable now checks requestContext.response.stream before constructing SseResponse and throws Managed SSE requires adapter-provided response.stream support. instead of returning true (silent handling). The failure flows through the standard dispatch error path (request error observers and the configured error response writer).
• packages/http/src/dispatch/dispatcher.test.ts: Added regression test rejects managed SSE AsyncIterable sources when the adapter exposes no response stream asserting the dispatcher commits a 500 response and notifies request error observers with the unsupported-stream message.
• packages/http/README.md / packages/http/README.ko.md: Documented that managed SSE requires an adapter that exposes FrameworkResponse.stream and that the dispatcher rejects the managed async iterable before marking the response handled when stream support is absent.
• .changeset/issue-2734-reject-managed-sse-without-stream.md: @fluojs/http patch changeset.

Testing

• pnpm --filter @fluojs/http typecheck — passed.
• pnpm --filter @fluojs/http test — 223 tests passed (83 dispatcher tests including the new regression test).
• pnpm verify:platform-consistency-governance — passed.
• pnpm verify:public-export-tsdoc (changed mode) — passed.

Release impact

• This PR has consumer-visible release impact and includes a changeset.

Public export documentation

• No public exports were added or changed; existing TSDoc is unaffected. The change is an internal dispatcher behavior correction.

Behavioral contract

• No documented behavioral contracts were removed without migration notes.
• New behavioral contracts are documented in the affected package README.
• Intentional limitations are explicitly stated (not silently removed).
• Runtime invariants are covered by regression tests.

Platform consistency governance (SSOT)

• SSOT English/Korean mirror structure remains synchronized for changed governance docs (README.md and README.ko.md updated together).
• No platform contract docs changed; companion updates not applicable.
• No package README alignment/conformance claims added beyond the documented stream support requirement, which is backed by the new dispatcher regression test.