fix(everything): close streamable HTTP transports on SIGINT

[dev-willbird1936]

Description

Fix Streamable HTTP shutdown so SIGINT closes every active transport. The session registry is a Map, but the previous for-in loop never enumerated its entries.

Root cause

Shutdown cleanup did not iterate Map entries, so active transports could remain registered and their close hooks were skipped.

Testing

• Focused Vitest regression with two active transports: 1/1 passed.
• Full Everything Vitest suite: 108/108 passed across 6 files.
• Fresh recheck on 2026-07-15 repeated the focused test (1/1), full package suite (108/108), and package build successfully.
• Everything package build and root TypeScript workspace build: passed.
• LLM-client integration was not exercised; tests use the transport implementation directly.

This is an ordinary correctness/resource-cleanup fix, not a security report. Hosted CI and maintainer review remain pending.