Assign `alias` to `validation_alias` and `serialization_alias` in `Body` and `Param`

This is how it's done in Pydantic's Field: 407934af10/pydantic/fields.py (L1346-L1350)

fastapi/params.py

@@ -115,6 +115,10 @@ class Param(FieldInfo):  # type: ignore[misc]
         else:
             kwargs["deprecated"] = deprecated
         if PYDANTIC_V2:
+            if serialization_alias in (_Unset, None) and isinstance(alias, str):
+                serialization_alias = alias
+            if validation_alias in (_Unset, None):
+                validation_alias = alias
             kwargs.update(
                 {
                     "annotation": annotation,
@@ -571,6 +575,10 @@ class Body(FieldInfo):  # type: ignore[misc]
         else:
             kwargs["deprecated"] = deprecated
         if PYDANTIC_V2:
+            if serialization_alias in (_Unset, None) and isinstance(alias, str):
+                serialization_alias = alias
+            if validation_alias in (_Unset, None):
+                validation_alias = alias
             kwargs.update(
                 {
                     "annotation": annotation,