Add SSE/RPC path and external_url params to filter chain factory (#215)

Extend HttpSseFilterChainFactory's constructor with three optional
server-side parameters so McpServer can propagate its configured
endpoint paths down into the filter chain without the factory having
to reach back into McpServerConfig:

- sse_path:     server-side SSE endpoint (matches McpServerConfig
                http_sse_path, default "/sse")
- rpc_path:     server-side JSON-RPC endpoint (default "/mcp")
- external_url: absolute URL the server is reachable at from the
                client's perspective; used to build the endpoint-event
                callback URL advertised on GET /sse. Empty means derive
                the URL from the incoming Host header.

Defaulted so existing callers (tests, client-mode construction,
example apps that only care about http_path/http_host) keep compiling
unchanged. The factory does not consume these fields yet — that wiring
lands in a follow-up commit once the SSE server transport itself goes
in on top of this stack.

Author: gophergogo

include/mcp/filter/http_sse_filter_chain_factory.h

@@ -66,19 +66,33 @@ class HttpSseFilterChainFactory : public network::FilterChainFactory {
    * @param http_host HTTP Host header value for client mode
    * @param use_sse True for SSE mode (GET /sse first), false for Streamable
    * HTTP (direct POST)
+   * @param sse_path Server-side SSE endpoint path (e.g., "/sse"). Only
+   *                 meaningful when is_server=true.
+   * @param rpc_path Server-side JSON-RPC endpoint path (e.g., "/mcp"). Only
+   *                 meaningful when is_server=true.
+   * @param external_url Absolute URL the server is reachable at from the
+   *                 client's perspective. Used to build the endpoint-event
+   *                 callback URL advertised on GET /sse. Leave empty to
+   *                 derive the URL from the incoming Host header.
    */
   HttpSseFilterChainFactory(event::Dispatcher& dispatcher,
                             McpProtocolCallbacks& message_callbacks,
                             bool is_server = true,
                             const std::string& http_path = "/rpc",
                             const std::string& http_host = "localhost",
-                            bool use_sse = true)
+                            bool use_sse = true,
+                            const std::string& sse_path = "/sse",
+                            const std::string& rpc_path = "/mcp",
+                            const std::string& external_url = "")
       : dispatcher_(dispatcher),
         message_callbacks_(message_callbacks),
         is_server_(is_server),
         http_path_(http_path),
         http_host_(http_host),
-        use_sse_(use_sse) {}
+        use_sse_(use_sse),
+        sse_path_(sse_path),
+        rpc_path_(rpc_path),
+        external_url_(external_url) {}
 
   /**
    * Create filter chain for the connection
@@ -167,9 +181,12 @@ class HttpSseFilterChainFactory : public network::FilterChainFactory {
   event::Dispatcher& dispatcher_;
   McpProtocolCallbacks& message_callbacks_;
   bool is_server_;
-  std::string http_path_;  // HTTP request path for client mode
-  std::string http_host_;  // HTTP Host header for client mode
-  bool use_sse_;           // True for SSE mode, false for Streamable HTTP
+  std::string http_path_;     // HTTP request path for client mode
+  std::string http_host_;     // HTTP Host header for client mode
+  bool use_sse_;              // True for SSE mode, false for Streamable HTTP
+  std::string sse_path_;      // Server-side SSE endpoint path (e.g., "/sse")
+  std::string rpc_path_;      // Server-side JSON-RPC endpoint path (e.g., "/mcp")
+  std::string external_url_;  // External URL for absolute SSE callback URLs
   mutable bool enable_metrics_ = true;  // Enable metrics by default
 
   // Store filters for lifetime management