[202405][GCU] Don't remove all entries in a table when just adding/modifying keys within an existing table

What I did
Fixed an issue in config apply-patch where adding a new entry to a table (e.g., INTERFACE) incorrectly removes all existing entries in that table before adding them back, causing unnecessary disruption.

Fixes: sonic-net/sonic-buildimage#24464 for 202405-msft

How I did it
Made two changes to generic_config_updater/patch_sorter.py:

KeyLevelMoveGenerator: Added check to prevent deleting entire table when it has a single key if that table will continue to exist in the target config. Now only deletes the whole table when table not in diff.target_config.
UpperLevelMoveExtender: Added logic to prevent key-level ADD/REPLACE operations from being extended to table-level operations when the table exists in both current and target configs. This keeps operations at the key level, avoiding unnecessary removal of unaffected entries.
How to verify it
Create a config with an INTERFACE table containing one entry (e.g., Ethernet0)
Apply a patch that adds a new entry (e.g., Ethernet10) to the INTERFACE table
Verify that only the new entry is added and the existing Ethernet0 entry is NOT removed during the operation
Check logs to confirm no REMOVE operations are generated for unaffected entries

Author: arlakshm

generic_config_updater/patch_sorter.py

@@ -1075,7 +1075,8 @@ def generate(self, diff):
         for tokens in self._get_non_existing_keys_tokens(diff.current_config, diff.target_config):
             table = tokens[0]
             # if table has a single key, delete the whole table because empty tables are not allowed in ConfigDB
-            if len(diff.current_config[table]) == 1:
+            # BUT only if the table won't exist in target config (i.e., not adding new keys to it)
+            if len(diff.current_config[table]) == 1 and table not in diff.target_config:
                 yield JsonMove(diff, OperationType.REMOVE, [table])
             else:
                 yield JsonMove(diff, OperationType.REMOVE, tokens)
@@ -1477,6 +1478,18 @@ def extend(self, move, diff):
             return
 
         upper_current_tokens = move.current_config_tokens[:-1]
+
+        # Don't extend key-level ADD/REPLACE operations to table-level when just adding/modifying keys within an existing table
+        # This prevents incorrectly replacing/removing entire tables when only adding/updating individual keys
+        # We still allow REMOVE operations to be extended to handle dependency cleanup properly
+        if len(upper_current_tokens) == 1:  # This would be a table-level operation
+            table = upper_current_tokens[0]
+            # If table exists in both current and target, don't extend ADD/REPLACE to table level
+            # The key-level operation is sufficient for additions/modifications
+            if table in diff.current_config and table in diff.target_config:
+                if move.op_type in [OperationType.ADD, OperationType.REPLACE]:
+                    return
+
         operation_type = self._get_upper_operation(upper_current_tokens, diff)
 
         upper_target_tokens = None