Fix tags getting removed after / during bank sync (#634)

* fix: Preserve transaction tags during rule application

When rules set tags, they now ADD to existing tags instead of replacing
them. This fixes issue #518 where tags were being removed during bank sync.

The root cause was that SetTransactionTags called enrich_attribute with
just the single tag from the rule, which replaced all existing tags.
Now it merges the new tag with existing tags using .uniq to prevent
duplicates.

This preserves:
- User-applied tags that shouldn't be overwritten by rules
- Tags from other rules when multiple rules match the same transaction
- Tags set during previous syncs

* fix: Add nil guard for tag in SetTransactionTags

Return early with 0 if the tag is not found, preventing NoMethodError
when find_by_id returns nil. This matches the pattern used in
SetTransactionMerchant.

---------

Co-authored-by: Claude <noreply@anthropic.com>

app/models/rule/action_executor/set_transaction_tags.rb

@@ -9,6 +9,7 @@ class Rule::ActionExecutor::SetTransactionTags < Rule::ActionExecutor
 
   def execute(transaction_scope, value: nil, ignore_attribute_locks: false, rule_run: nil)
     tag = family.tags.find_by_id(value)
+    return 0 unless tag
 
     scope = transaction_scope
 
@@ -17,9 +18,14 @@ class Rule::ActionExecutor::SetTransactionTags < Rule::ActionExecutor
     end
 
     count_modified_resources(scope) do |txn|
+      # Merge the new tag with existing tags instead of replacing them
+      # This preserves tags set by users or other rules
+      existing_tag_ids = txn.tag_ids || []
+      merged_tag_ids = (existing_tag_ids + [ tag.id ]).uniq
+
       txn.enrich_attribute(
         :tag_ids,
-        [ tag.id ],
+        merged_tag_ids,
         source: "rule"
       )
     end