feat: implement storeGroupValue and getGroupValue with unified locator

- API Refactoring: Extracted GroupValueLocator input type to unify parameters for storeGroupValue (mutation) and getGroupValue (query).

- Data Retrieval: Implemented getGroupValue query to allow direct retrieval of JSON values stored in host groups via Zabbix Trapper items.

- Enhanced Logic: Added ZabbixGetGroupValueRequest to fetch latest history values for group-associated items.

- Improved Verification: Updated the regression suite (REG-STORE) to include a full 'Store-Update-Retrieve' verification cycle.

- Documentation:

    - Updated docs/howtos/cookbook.md recipes to use the new locator structure and getGroupValue for verification.

    - Updated sample query files (docs/queries/) with corrected variables and verification queries.

- Tests:

    - Added unit and integration tests for getGroupValue.

    - Updated existing tests to match the refactored storeGroupValue schema.

- Verification: Verified 100% pass rate for all 16 regression steps and all unit/integration tests.

schema/queries.graphql

@@ -169,5 +169,15 @@ type Query {
         """Wildcard name pattern for filtering template groups."""
         name_pattern: String
     ): [HostGroup]
+
+    """
+    Retrieves the last value stored with `storeGroupValue`.
+    
+    Authentication: Requires `zbx_session` cookie or `zabbix-auth-token` header.
+    """
+    getGroupValue(
+        """Parameters to locate the stored value."""
+        locator: GroupValueLocator!
+    ): JSONObject
 }