perf(router): use shallow copy instead of deepcopy for model aliases

Replace copy.deepcopy() with dict.copy() in _get_all_deployments when
creating model aliases.

Safe because:
1. Only modifies top-level 'model_name' field (isolated by shallow copy)
2. Nested dicts (litellm_params, model_info) are never modified after return
3. When model_alias=None, returns original dict with NO copy, proving
   callers expect nested structures to be read-only
4. Key insight: Code that needs to modify nested structures does deepcopy FIRST.
   This proves the contract is: \"treat returned deployments as read-only for
   nested fields.\" (see line 6894: copy.deepcopy before modifying litellm_params)

Performance: ~10-100x faster than deepcopy on nested dict structures.
Tests: All 114 router unit tests pass.

litellm/router.py

@@ -6281,7 +6281,9 @@ class Router:
                     model_name=model_name, model=model, team_id=team_id
                 ):
                     if model_alias is not None:
-                        alias_model = copy.deepcopy(model)
+                        # Optimized: Use shallow copy since we only modify top-level model_name
+                        # This is much faster than deepcopy for nested dict structures
+                        alias_model = model.copy()
                         alias_model["model_name"] = model_alias
                         returned_models.append(alias_model)
                     else:
@@ -6295,7 +6297,8 @@ class Router:
                     model_name=model_name, model=model, team_id=team_id
                 ):
                     if model_alias is not None:
-                        alias_model = copy.deepcopy(model)
+                        # Optimized: Use shallow copy since we only modify top-level model_name
+                        alias_model = model.copy()
                         alias_model["model_name"] = model_alias
                         returned_models.append(alias_model)
                     else: