Address code review feedback for HTTP client and server

Based on code review feedback:

Client improvements:
- Remove redundant capacity check in http_client_get_builder
  (already verified num_conns < capacity)
- Fix generation counter - no longer reset to zero when allocating
  connections, only incremented on request send
- Add HTTP_Method parameter to http_request_builder_url instead
  of hardcoding GET method
- Free connection resources when http_request_builder_send fails
  (invalid host mode or socket_connect failure)

Server bug fix:
- Fix ready queue indexing in http_server_next_request
  Changed: &server->conns[server->ready_head]
  To: &server->conns[server->ready[server->ready_head]]
  This bug would have caused incorrect connection retrieval

All changes maintain consistency with the codebase style and
improve robustness of error handling.

src/client.h

@@ -36,6 +36,9 @@ typedef struct {
     // Data being sent to the server
     ByteQueue output;
 
+    // HTTP method for the request
+    HTTP_Method method;
+
     // Parsed URL for connection establishment
     HTTP_URL url;
 
@@ -106,9 +109,10 @@ typedef struct {
 int http_client_get_builder(HTTP_Client *client,
     HTTP_Response *response, HTTP_RequestBuilder *builder);
 
-// Set the URL of the current request. This is the first
+// Set the method and URL of the current request. This is the first
 // function of the request builder that the user must call.
-void http_request_builder_url(HTTP_RequestBuilder builder, HTTP_String url);
+void http_request_builder_url(HTTP_RequestBuilder builder,
+    HTTP_Method method, HTTP_String url);
 
 // After the URL, the user may set zero or more headers.
 void http_request_builder_header(HTTP_RequestBuilder builder, HTTP_String str);