Refactor backport table generation in PR description validation

This commit enhances the `generate_backport_table` function in `validate_pr_description.py` to collect and sort unique branches from the input. The changes ensure that the backporting URL is generated for all unique branches, improving the accuracy and clarity of the backport table in PR descriptions.

Key changes:
- Updated logic to collect unique branches from multiple entries.
- Sorted branches for consistent output.
- Modified the backport button label for clarity.

Author: naspirato

.github/actions/validate_pr_description/validate_pr_description.py

@@ -191,8 +191,18 @@ def generate_backport_table(pr_number: int, app_domain: str) -> str:
 
         rows.append(f"| [![▶ {branch}](https://img.shields.io/badge/%E2%96%B6_{branch.replace('-', '_')}-4caf50?style=flat-square)]({url}) [![⚙️](https://img.shields.io/badge/%E2%9A%99%EF%B8%8F-ff9800?style=flat-square)]({url_ui}) |")
 
-    # Generate URL for backporting multiple branches
-    all_branches = ",".join(branches)
+    # Collect all unique branches from all entries (each entry may contain multiple branches separated by comma)
+    all_unique_branches = set()
+    for branch_entry in branches:
+        # Split by comma and strip whitespace
+        branch_list = [b.strip() for b in branch_entry.split(',')]
+        all_unique_branches.update(branch_list)
+    
+    # Sort for consistent output
+    all_unique_branches_sorted = sorted(all_unique_branches)
+    all_branches = ",".join(all_unique_branches_sorted)
+    
+    # Generate URL for backporting all unique branches
     params_multiple = {
         "owner": owner,
         "repo": repo,
@@ -212,7 +222,7 @@ def generate_backport_table(pr_number: int, app_domain: str) -> str:
     table += "|----------|\n"
     table += "\n".join(rows)
     table += "\n\n"
-    table += f"[![⚙️ Backport multiple branches](https://img.shields.io/badge/%E2%9A%99%EF%B8%8F_Backport_multiple_branches-2196F3?style=flat-square)]({url_multiple_ui})"
+    table += f"[![⚙️ Backport (custom)](https://img.shields.io/badge/%E2%9A%99%EF%B8%8F_Backport_%28custom%29-2196F3?style=flat-square)]({url_multiple_ui})"
     return table
 
 def get_legend() -> str: