fix(everything): return errors for missing SSE sessions

[dev-willbird1936]

Description

Return clear 404 JSON responses when the deprecated Everything SSE transport receives an unknown or missing session ID. The affected routes previously logged and left requests unresolved, or dereferenced a failed lookup.

Root cause

The deprecated SSE transport did not consistently validate session lookup before handling GET /sse and POST /message requests.

Testing

• Vitest HTTP-level regressions for unknown GET /sse and POST /message sessions: 2/2 passed.
• Fresh Everything suite recheck on 2026-07-15: 109/109 tests passed across 6 files.
• Everything package build passed.
• Touched-file Prettier and git diff --check passed.
• LLM-client integration was not exercised; tests use HTTP-level transport requests only.

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