fix(security): declare 204 response schema on PATCH /session

[birme]

Summary

• The PATCH /session/:sessionId handler sends reply.code(204).send() on success, but the TypeBox response schema only declared 200: Type.String(). Because 204 was undeclared, Fastify applied the wrong serializer and skipped response validation for the success path (a validation bypass).
• Replaced the incorrect 200: Type.String() entry with 204: Type.Null(), keeping the existing 400/500 entries. Minimal one-line schema change.
• Added a regression test asserting PATCH /session/:sessionId returns 204 with an empty body on success (backend tests mock ./log).

Test plan

• Tests pass (npm test) — 244 passed
• TypeScript compiles (npm run typecheck)
• Lint clean (npm run lint) — 0 errors
• PATCH /session returns 204 and is now schema-validated

Closes #289

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com