test: add unit tests for InMemoryRuntimeStateStore put/get round trips (Issue #109)

[rafaio1]

Summary

Adds unit tests verifying InMemoryRuntimeStateStore put/get semantics including type preservation, overwrite behavior, null/zero handling, and key isolation.

Tests Added

• ✅ String and object values stored and retrieved correctly
• ✅ Undefined returned for non-existent keys
• ✅ Subsequent puts overwrite existing values
• ✅ Null and zero preserved as valid stored values
• ✅ Multiple keys isolated from each other

Verification

All 6 tests pass locally via npx vitest run tests/state/runtime-state-store.test.ts.

Closes #109