fix: reduced __inits__ overhead in 7% (#14689)

* fix: avoid redundant __init__ calls on hot path

Previously, imports on the request hot path caused __init__ to run
excessively for every request. This change ensures initialization
happens once, reducing cpu overhead.

* fix: remove redundant __init__ import

The current implementation no longer requires an import at the top of the function.

* fix: placed on core utils for future reuse

* test: add coverage & remove inline import

A general import-checking tool across all endpoints would be a large PR.
This commit focuses on a smaller, targeted fix for the discussed case.

* added import check to CI

tests/code_coverage_tests/test_chat_completion_imports.py

@@ -0,0 +1,43 @@
+## Tests that chat_completion endpoint has no imports inside function bodies
+## This is critical for performance optimization in the hot path
+
+import ast
+from pathlib import Path
+
+
+def test_chat_completion_no_imports():
+    """Test that chat_completion endpoint has no imports in function bodies."""
+    # Path to the proxy server file
+    proxy_server_path = Path(__file__).parent.parent.parent / "litellm" / "proxy" / "proxy_server.py"
+    
+    with open(proxy_server_path, 'r') as f:
+        content = f.read()
+    
+    # Parse the AST
+    tree = ast.parse(content)
+    
+    # Find the chat_completion function
+    chat_completion_func = None
+    for node in ast.walk(tree):
+        if (isinstance(node, ast.AsyncFunctionDef) and node.name == "chat_completion"):
+            chat_completion_func = node
+            break
+    
+    assert chat_completion_func is not None, "chat_completion function not found"
+    
+    # Check for imports inside the function body
+    import_violations = []
+    
+    for node in ast.walk(chat_completion_func):
+        if isinstance(node, (ast.Import, ast.ImportFrom)):
+            # Get line number
+            line_num = node.lineno
+            import_violations.append(line_num)
+    
+    # Assert no import violations found
+    if import_violations:
+        print(f"Found {len(import_violations)} import violations in chat_completion endpoint:")
+        for line_num in import_violations:
+            print(f"  - Line {line_num}: Import statement found")
+        print("\nchat_completion endpoint should not contain imports for optimal performance.")
+        raise Exception("Import violations found in chat_completion endpoint")