fix(params): display const value in parameter description and Try It form (#1367)

- ParamsItem now renders "Constant value: <X>" for parameters whose schema
  uses const, reusing the existing CONSTANT_VALUE translation ID
- getSchemaEnum now treats schema.const as a single-value enum, routing
  const parameters to the existing ParamSelectFormItem dropdown instead
  of a free-form text input

Closes #1307

Author: sserrata

packages/docusaurus-theme-openapi-docs/src/theme/ApiExplorer/ParamOptions/index.tsx

@@ -48,6 +48,11 @@ export function getSchemaEnum(schema: any): any[] | undefined {
     }
   }
 
+  // const is semantically a single-value enum
+  if (schema?.const !== undefined) {
+    return [schema.const];
+  }
+
   return undefined;
 }
 

packages/docusaurus-theme-openapi-docs/src/theme/ParamsItem/index.tsx

@@ -102,10 +102,34 @@ function ParamsItem({ param, ...rest }: Props) {
     </span>
   ));
 
+  const constValue = schema?.const;
+
   const renderQualifier = guard(getQualifierMessage(schema), (qualifier) => (
     <Markdown>{qualifier}</Markdown>
   ));
 
+  function renderConstValue() {
+    if (constValue === undefined) {
+      return undefined;
+    }
+    const label = translate({
+      id: OPENAPI_SCHEMA_ITEM.CONSTANT_VALUE,
+      message: "Constant value:",
+    });
+    return (
+      <div>
+        <strong>{label} </strong>
+        <span>
+          <code>
+            {typeof constValue === "string"
+              ? constValue
+              : JSON.stringify(constValue)}
+          </code>
+        </span>
+      </div>
+    );
+  }
+
   const renderDescription = guard(description, (description) => (
     <Markdown>{description}</Markdown>
   ));
@@ -173,6 +197,7 @@ function ParamsItem({ param, ...rest }: Props) {
         {renderDeprecated}
       </span>
       {renderQualifier}
+      {renderConstValue()}
       {renderDescription}
       {renderEnumDescriptions}
       {renderDefaultValue()}