Add __str__ methods to validation error classes for better debugging

Added human-readable string representations to RequestValidationError and
WebSocketRequestValidationError classes, consistent with the existing
ResponseValidationError implementation. This improves developer experience
when debugging validation errors by providing clear, formatted error messages.

Changes:
- Add __str__ method to RequestValidationError
- Add __str__ method to WebSocketRequestValidationError
- Add comprehensive tests for the new string representations

The string format shows the number of errors and lists each error on a
separate line, making it easier to read error messages during debugging
and logging.

fastapi/exceptions.py

@@ -167,9 +167,25 @@ class RequestValidationError(ValidationException):
         super().__init__(errors)
         self.body = body
 
+    def __str__(self) -> str:
+        message = f"{len(self._errors)} validation error"
+        if len(self._errors) > 1:
+            message += "s"
+        message += " for request:\n"
+        for err in self._errors:
+            message += f"  {err}\n"
+        return message
+
 
 class WebSocketRequestValidationError(ValidationException):
-    pass
+    def __str__(self) -> str:
+        message = f"{len(self._errors)} validation error"
+        if len(self._errors) > 1:
+            message += "s"
+        message += " for WebSocket request:\n"
+        for err in self._errors:
+            message += f"  {err}\n"
+        return message
 
 
 class ResponseValidationError(ValidationException):

tests/test_validation_error_str.py

@@ -0,0 +1,65 @@
+"""Tests for __str__ methods of validation exceptions."""
+
+from fastapi.exceptions import (
+    RequestValidationError,
+    ResponseValidationError,
+    WebSocketRequestValidationError,
+)
+
+
+def test_request_validation_error_str_single_error():
+    """Test string representation of RequestValidationError with single error."""
+    errors = [{"type": "int_parsing", "loc": ["query", "id"], "msg": "Input should be a valid integer"}]
+    exc = RequestValidationError(errors)
+    result = str(exc)
+
+    assert "1 validation error for request:" in result
+    assert "{'type': 'int_parsing'" in result
+
+
+def test_request_validation_error_str_multiple_errors():
+    """Test string representation of RequestValidationError with multiple errors."""
+    errors = [
+        {"type": "int_parsing", "loc": ["query", "id"], "msg": "Input should be a valid integer"},
+        {"type": "missing", "loc": ["query", "name"], "msg": "Field required"},
+    ]
+    exc = RequestValidationError(errors)
+    result = str(exc)
+
+    assert "2 validation errors for request:" in result
+    assert "int_parsing" in result
+    assert "missing" in result
+
+
+def test_websocket_request_validation_error_str_single_error():
+    """Test string representation of WebSocketRequestValidationError with single error."""
+    errors = [{"type": "int_parsing", "loc": ["query", "id"], "msg": "Input should be a valid integer"}]
+    exc = WebSocketRequestValidationError(errors)
+    result = str(exc)
+
+    assert "1 validation error for WebSocket request:" in result
+    assert "{'type': 'int_parsing'" in result
+
+
+def test_websocket_request_validation_error_str_multiple_errors():
+    """Test string representation of WebSocketRequestValidationError with multiple errors."""
+    errors = [
+        {"type": "int_parsing", "loc": ["query", "id"], "msg": "Input should be a valid integer"},
+        {"type": "missing", "loc": ["query", "name"], "msg": "Field required"},
+    ]
+    exc = WebSocketRequestValidationError(errors)
+    result = str(exc)
+
+    assert "2 validation errors for WebSocket request:" in result
+    assert "int_parsing" in result
+    assert "missing" in result
+
+
+def test_response_validation_error_str_consistency():
+    """Test that ResponseValidationError has similar __str__ behavior."""
+    errors = [{"type": "int_parsing", "loc": ["response", "id"], "msg": "Input should be a valid integer"}]
+    exc = ResponseValidationError(errors)
+    result = str(exc)
+
+    assert "1 validation error" in result
+    assert "{'type': 'int_parsing'" in result