[Darwin] Guard CHIP source-file/line block under CHIP_CONFIG_ERROR_SOURCE

Per Copilot review on PR project-chip#72211:

- ChipError::GetFile() / GetLine() are only defined when CHIP is built with
  CHIP_CONFIG_ERROR_SOURCE; builds with that flag disabled fail to compile.
  Wrap the source-info userInfo block under the same #if so the keys are
  simply absent on those builds.

- Initialize legacyAdditionalUserInfo to nil at declaration. ARC zero-inits
  __strong locals already, but the explicit nil makes the contract clear.

Author: woody-apple

src/darwin/Framework/CHIP/MTRError.mm

@@ -84,7 +84,7 @@ + (NSError *)errorForCHIPErrorCode:(CHIP_ERROR)errorCode
 
     MTRErrorCode code;
     NSString * description;
-    NSDictionary * legacyAdditionalUserInfo;
+    NSDictionary * legacyAdditionalUserInfo = nil;
     switch (errorCode.AsInteger()) {
     case CHIP_ERROR_INVALID_STRING_LENGTH.AsInteger():
         code = MTRErrorCodeInvalidStringLength;
@@ -170,6 +170,7 @@ + (NSError *)errorForCHIPErrorCode:(CHIP_ERROR)errorCode
     // exposing build-host paths; only the basename is forwarded. Check both
     // forward and backward slashes so basename-stripping works on Windows builds
     // as well as Unix-style hosts.
+#if CHIP_CONFIG_ERROR_SOURCE
     const char * sourceFile = errorCode.GetFile();
     if (sourceFile != nullptr && sourceFile[0] != '\0') {
         const char * basename = strrchr(sourceFile, '/');
@@ -179,6 +180,7 @@ + (NSError *)errorForCHIPErrorCode:(CHIP_ERROR)errorCode
         userInfo[MTRCHIPErrorSourceFileKey] = @((basename != nullptr) ? basename + 1 : sourceFile);
         userInfo[MTRCHIPErrorSourceLineKey] = @(errorCode.GetLine());
     }
+#endif // CHIP_CONFIG_ERROR_SOURCE
 
     if (legacyAdditionalUserInfo) {
         [userInfo addEntriesFromDictionary:legacyAdditionalUserInfo];