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/api_commons.graphql

@@ -159,3 +159,27 @@ enum SortOrder {
     "Deliver values in descending order"
     desc
 }
+
+"""
+Input for locating a specific value stored within a host group.
+Used by both retrieval queries and storage mutations.
+"""
+input GroupValueLocator {
+    """ID of the target host group (either groupid or groupName is required)."""
+    groupid: Int
+    """Name of the target host group (either groupid or groupName is required)."""
+    groupName: String
+    """Name of the host to store/retrieve the value (optional). If not provided, valueType is used to find or create a storage host."""
+    host: String
+    """
+    The value for the "valueType" tag of the storage host. 
+    Mandatory if no host is provided. Used to identify the host within the group.
+    """
+    valueType: String
+    """Item ID if an existing item should be used."""
+    itemid: Int
+    """The technical key of the item."""
+    key: String!
+    """The visible name of the item (optional)."""
+    name: String
+}