Add shallow equality guards to MessageBlockStore to prevent unnecessary renders

[nordicnode]

Add shallow equality guards to MessageBlockStore to prevent unnecessary renders

Summary

• In cli/src/state/message-block-store.ts, added shallow equality guards to setContext and setCallbacks to prevent state reference churn and eliminate spurious notifications to store subscribers and context consumers.
• Previously, setContext (triggered on every layout effect in <Chat />) drafted and replaced state.context with a new object reference even when all property values were identical. Because Zustand notifies store subscribers whenever a new state reference is created by Immer, this triggered re-render passes for any components and listeners consuming the store or context directly.
• In setContext, added a key-by-key shallow equality guard for merged properties.
• In setCallbacks, respected full replacement semantics by checking key set length before checking individual values, ensuring that omitted callback keys trigger replacement rather than being silently preserved.
• Exported and shared a single EMPTY_RESPONSE_ADS module constant between initialContext and chat.tsx, ensuring stable object reference identity when ads are disabled with zero allocation.
• Added unit test suite in cli/src/state/__tests__/message-block-store.test.ts verifying that subscribers fire only on true mutations, are suppressed on identical updates, and handle omitted keys in setCallbacks.

Test plan

[✓] bun test --config=/dev/null src/state/__tests__/message-block-store.test.ts — 4 pass, 0 fail
[✓] bun run --cwd cli typecheck — 0 errors
[✓] PR hygiene check passed