Add provider contract tests for MemoryStore and RuntimeStateStore implementations

[David-patrick-chuks]

Summary

Create shared contract tests for storage abstractions so new adapters can be validated consistently.

Context

Contributor-added storage implementations will become hard to trust if each backend invents slightly different behavior.

Scope

• Define reusable contract tests for MemoryStore.
• Define reusable contract tests for RuntimeStateStore.
• Ensure in-memory and file-backed implementations can run against the same test suites.
• Document required semantics for adapter authors.

Acceptance Criteria

• Shared contract test suites exist.
• At least two implementations run against them where applicable.
• Contract expectations are documented.
• CI runs the contract suites.

Out of Scope

• Database-specific stress tests.
• Distributed consistency guarantees.
• Performance comparisons.

Dependencies

• Define a durable MemoryStore contract and add a file-backed reference adapter #5
• Add namespaced runtime state keys and optional TTL semantics to RuntimeStateStore #6

Suggested Labels

tests, memory, state