fix(knowledge): exit when server startup fails - #1141

Summary

• exit with status 1 when MemoryKnowledge fails during startup
• add a regression test for an invalid SQLite path while Langfuse telemetry is active

Problem

The existing handler only assigned process.exitCode. If telemetry had already created active handles, the HTTP listener never started but the process stayed alive.

Testing

• pnpm test
• pnpm build
• Ubuntu, Node.js 22: invalid KNOWLEDGE_DB_PATH with Langfuse enabled exits with status 1

pnpm typecheck currently fails in the unchanged src/middleware/response-envelope.ts:47 because Promise<string> is assigned to string.

Closes #1140