test(server): migrate missing patch diff test

[kitlangton]

Summary

• Migrates the missing-patch session diff regression test to the Effect-native testEffect(...).instance(...) runner style.
• Uses the test instance directory and scoped session acquire/release instead of WithInstance.provide and nested Effect.runPromise calls.

Tests

• bun run test test/server/session-diff-missing-patch.test.ts
• bun typecheck