Merged PR 2242: Refactor: New Appeal flow structural cleanup (Slices 1–8, no behaviour change)

## Summary

This PR merges the completed new-appeal refactor stream (Slices 1–8) into SIPS-Development.

This refactor was executed in controlled slices with regression validation at each step.

## Scope

Structural and readability improvements only:
- XML/form derivation extraction
- payload and file helper extraction
- side-effect facade introduction
- BuildSection and BuildCheckSection decomposition
- BuildCheckRow formatter map refactor
- nested prop boundary cleanup
- start-flow cleanup (CreateCase / AboutYou)

## Behaviour

No intended behavioural changes.

The following were explicitly preserved:
- S78 journey behaviour
- payload shapes and field names/ids
- HASCAS mapping logic
- appellant/agent branching
- validation rules and messages
- redirect/query parameters (`lpa`, `apt`, `id`)
- navigation and side-effect sequencing
- EN/CY output parity

## Validation

Full regression has been performed on:
- start flow (CreateCase / AboutYou)
- save and resume flows
- file upload handling
- check answers rendering
- submit/finalisation sequence
- confirmation flow
- CRM insertion path
- EN/CY parity

Additional checks:
- docsOffline branch behaviour
- completion and partial-save email paths
- negative-path validation scenarios
- lint (warnings baseline unchanged)

## Risk

Low:
- changes are structural only
- no business logic changes
- no contract changes

## Rollback

Safe rollback via reverting this merge commit.

## Notes

This refactor reduces coupling and prepares the new-appeal flow for future appeal-type expansion.

Related work items: #22570, #22576, #22577, #22583, #22586, #22587, #22588, #22590

lib/newappeal/fileListHelpers.js

@@ -0,0 +1,28 @@
+export const mergeWithExistingFilesList = (buildAppealFilesArray, values) => {
+    return values.hasOwnProperty("filesList")
+        ? (buildAppealFilesArray.concat(values.filesList),
+          buildAppealFilesArray.filter(function (item, idx) {
+              return item.name;
+          }))
+        : buildAppealFilesArray;
+};
+
+export const removeDuplicatesByKey = (arr, key) => {
+    const seen = new Set();
+    return arr.filter((item) => {
+        const value = item[key];
+        if (seen.has(value)) {
+            return false; // Duplicate found
+        } else {
+            seen.add(value); // Add value to the set
+            return true; // Keep the item
+        }
+    });
+};
+
+export const dedupeFilesListByName = (filesList) => {
+    return filesList.filter(
+        (value, index, self) =>
+            index === self.findIndex((t) => t.name === value.name)
+    );
+};