everything: Streamable HTTP SIGINT handler never closes a session (for...in over a Map) #4983

Problem

The shutdown handler in transports/streamableHttp.ts iterates the session Map with for...in, which yields no entries for a Map. On Ctrl-C the process logs and exits 0 without closing any transport, so open sessions are never cleaned up.

Where it is pinned

src/everything/__tests__/streamable-http.test.ts: "shutdown: logs and exits 0 without closing any session", added in #4978 (Wave 1 of #4857). Each of these tests carries a KNOWN BUG marker: it asserts the current, wrong behavior, so the fix has to change it.

Expected

Shutdown closes every open transport (iterate with for...of over the map's values) before exiting.

Done when

• The behavior above is fixed, in the legacy-era (2025-11-25) server on SDK 1.x.
• The pinning tests assert the correct behavior, and their KNOWN BUG markers are gone.
• The per-file 90% coverage gate still passes.