[compute_ctl] User correct HTTP code in the /configure errors (#6017)

It was using `PRECONDITION_FAILED` for errors during `ComputeSpec` to
`ParsedSpec` conversion, but this disobeys the OpenAPI spec [1] and
correct code should be `BAD_REQUEST` for any spec processing errors.

While on it, I also noticed that `compute_ctl` OpenAPI spec has an
invalid format and fixed it.

[1] https://github.com/neondatabase/neon/blob/fd81945a60dff25fea65a50f2c8cd6e4c955fba4/compute_tools/src/http/openapi_spec.yaml#L119-L120

compute_tools/src/http/api.rs

@@ -227,7 +227,7 @@ async fn handle_configure_request(
 
         let parsed_spec = match ParsedSpec::try_from(spec) {
             Ok(ps) => ps,
-            Err(msg) => return Err((msg, StatusCode::PRECONDITION_FAILED)),
+            Err(msg) => return Err((msg, StatusCode::BAD_REQUEST)),
         };
 
         // XXX: wrap state update under lock in code blocks. Otherwise,

compute_tools/src/http/openapi_spec.yaml

@@ -156,17 +156,17 @@ paths:
                 description: Error text or 'OK' if download succeeded.
                 example: "OK"
         400:
-        description: Request is invalid.
-        content:
-          application/json:
-            schema:
-              $ref: "#/components/schemas/GenericError"
+          description: Request is invalid.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/GenericError"
         500:
-        description: Extension download request failed.
-        content:
-          application/json:
-            schema:
-              $ref: "#/components/schemas/GenericError"
+          description: Extension download request failed.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/GenericError"
 
 components:
   securitySchemes: