perf(ServiceProviderRegistry): getProviderPayee helper for dataSetCreated (#249)

## Description

Implements a helper function to fetch only the payee address for service
providers, reducing the access footprint of `CreateDataSet`.

## Problem

Currently, `FilecoinWarmStorageService.dataSetCreated()` calls
`getProvider(providerId)` which reads the entire `ServiceProviderInfo`
struct from storage, including expensive string fields (`name` and
`description`), when only the `payee` address is needed.

## Solution

- **Added `getProviderPayee(uint256 providerId)`** in
`ServiceProviderRegistry.sol`
  - Returns only the payee address directly from storage
  - Uses `providerExists` modifier for safety

- **Updated `FilecoinWarmStorageService.sol`**
- Replaced `getProvider()` call with `getProviderPayee()` in
`dataSetCreated()`
  - Eliminates intermediate struct creation and string reads

- **Added unit tests**
- `testGetProviderPayeeReturnsCorrectAddress()` - verifies correct payee
returned
- `testGetProviderPayeeRevertsForInvalidProviderId()` - verifies revert
behavior

Fixes #248

Author: Chaitu-Tatipamula

service_contracts/abi/ServiceProviderRegistry.abi.json

@@ -824,6 +824,25 @@
     ],
     "stateMutability": "view"
   },
+  {
+    "type": "function",
+    "name": "getProviderPayee",
+    "inputs": [
+      {
+        "name": "providerId",
+        "type": "uint256",
+        "internalType": "uint256"
+      }
+    ],
+    "outputs": [
+      {
+        "name": "payee",
+        "type": "address",
+        "internalType": "address"
+      }
+    ],
+    "stateMutability": "view"
+  },
   {
     "type": "function",
     "name": "getProvidersByProductType",

service_contracts/src/FilecoinWarmStorageService.sol

@@ -489,9 +489,7 @@ contract FilecoinWarmStorageService is
         // Check if provider is approved
         require(approvedProviders[providerId], Errors.ProviderNotApproved(serviceProvider, providerId));
 
-        ServiceProviderRegistry.ServiceProviderInfoView memory providerInfo =
-            serviceProviderRegistry.getProvider(providerId);
-        address payee = providerInfo.info.payee;
+        address payee = serviceProviderRegistry.getProviderPayee(providerId);
 
         uint256 clientDataSetId = clientDataSetIds[createData.payer]++;
         clientDataSets[createData.payer].push(dataSetId);

service_contracts/src/ServiceProviderRegistry.sol

@@ -484,6 +484,13 @@ contract ServiceProviderRegistry is
         return ServiceProviderInfoView({providerId: providerId, info: provider});
     }
 
+    /// @notice Get only the payee address for a provider
+    /// @param providerId The ID of the provider
+    /// @return payee The payee address
+    function getProviderPayee(uint256 providerId) external view providerExists(providerId) returns (address payee) {
+        return providers[providerId].payee;
+    }
+
     /// @notice Get product data for a specific product type
     /// @param providerId The ID of the provider
     /// @param productType The type of product to retrieve

service_contracts/test/ServiceProviderRegistry.t.sol

@@ -308,4 +308,55 @@ contract ServiceProviderRegistryTest is Test {
         registry.transferOwnership(user1);
         assertEq(registry.owner(), user1, "Service provider should be transferred");
     }
+
+    function testGetProviderPayeeReturnsCorrectAddress() public {
+        // Give user1 some ETH for registration fee
+        vm.deal(user1, 10 ether);
+
+        // Prepare PDP data
+        ServiceProviderRegistryStorage.PDPOffering memory pdpData = ServiceProviderRegistryStorage.PDPOffering({
+            serviceURL: "https://example.com",
+            minPieceSizeInBytes: 1024,
+            maxPieceSizeInBytes: 1024 * 1024,
+            ipniPiece: true,
+            ipniIpfs: false,
+            storagePricePerTibPerMonth: 500000000000000000, // 0.5 FIL per TiB per month
+            minProvingPeriodInEpochs: 2880,
+            location: "US-East",
+            paymentTokenAddress: IERC20(address(0)) // Payment in FIL
+        });
+
+        // Encode PDP data
+        bytes memory encodedData = abi.encode(pdpData);
+
+        // Empty capability arrays
+        string[] memory emptyKeys = new string[](0);
+        string[] memory emptyValues = new string[](0);
+
+        // Register provider with user2 as payee
+        vm.prank(user1);
+        uint256 providerId = registry.registerProvider{value: 5 ether}(
+            user2,
+            "Provider One",
+            "Test provider description",
+            ServiceProviderRegistryStorage.ProductType.PDP,
+            encodedData,
+            emptyKeys,
+            emptyValues
+        );
+
+        // Verify helper returns the payee address
+        address payee = registry.getProviderPayee(providerId);
+        assertEq(payee, user2, "getProviderPayee should return the registered payee");
+    }
+
+    function testGetProviderPayeeRevertsForInvalidProviderId() public {
+        // 0 is invalid provider ID; expect revert due to providerExists modifier
+        vm.expectRevert("Provider does not exist");
+        registry.getProviderPayee(0);
+
+        // Non-existent but non-zero ID should also revert
+        vm.expectRevert("Provider does not exist");
+        registry.getProviderPayee(1);
+    }
 }