Deprecate Builder.customizeRequest() in HttpClientSseClientTransport and HttpClientStreamableHttpTransport

customizeRequest() executes its consumer once at build time, freezing
headers into the shared requestBuilder. This silently breaks OAuth
token refresh scenarios where the Authorization header needs to be
updated after the transport is built.

Add @deprecated and update Javadoc to clarify the build-time-only
semantics and guide users toward httpRequestCustomizer() or
asyncHttpRequestCustomizer() which run on every request.

Closes #788

Author: gyeo009

mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java

@@ -241,10 +241,24 @@ public Builder requestBuilder(HttpRequest.Builder requestBuilder) {
 		}
 
 		/**
-		 * Customizes the HTTP client builder.
+		 * Applies the given consumer to the shared {@link HttpRequest.Builder} <b>once,
+		 * at build time</b>. Any headers set here (e.g., {@code Authorization}) are
+		 * frozen into the template and copied to every subsequent request via
+		 * {@code requestBuilder.copy()}. They <b>cannot be updated</b> after the
+		 * transport is built.
+		 * <p>
+		 * For dynamic, per-request customization (e.g., OAuth token refresh), use
+		 * {@link #httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)} or
+		 * {@link #asyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer)}
+		 * instead.
 		 * @param requestCustomizer the consumer to customize the HTTP request builder
 		 * @return this builder
+		 * @deprecated Use
+		 * {@link #httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)} or
+		 * {@link #asyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer)} which
+		 * run on every request and support dynamic headers.
 		 */
+		@Deprecated
 		public Builder customizeRequest(final Consumer<HttpRequest.Builder> requestCustomizer) {
 			Assert.notNull(requestCustomizer, "requestCustomizer must not be null");
 			requestCustomizer.accept(requestBuilder);

mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java

@@ -738,10 +738,24 @@ public Builder requestBuilder(HttpRequest.Builder requestBuilder) {
 		}
 
 		/**
-		 * Customizes the HTTP client builder.
+		 * Applies the given consumer to the shared {@link HttpRequest.Builder} <b>once,
+		 * at build time</b>. Any headers set here (e.g., {@code Authorization}) are
+		 * frozen into the template and copied to every subsequent request via
+		 * {@code requestBuilder.copy()}. They <b>cannot be updated</b> after the
+		 * transport is built.
+		 * <p>
+		 * For dynamic, per-request customization (e.g., OAuth token refresh), use
+		 * {@link #httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)} or
+		 * {@link #asyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer)}
+		 * instead.
 		 * @param requestCustomizer the consumer to customize the HTTP request builder
 		 * @return this builder
+		 * @deprecated Use
+		 * {@link #httpRequestCustomizer(McpSyncHttpClientRequestCustomizer)} or
+		 * {@link #asyncHttpRequestCustomizer(McpAsyncHttpClientRequestCustomizer)} which
+		 * run on every request and support dynamic headers.
 		 */
+		@Deprecated
 		public Builder customizeRequest(final Consumer<HttpRequest.Builder> requestCustomizer) {
 			Assert.notNull(requestCustomizer, "requestCustomizer must not be null");
 			requestCustomizer.accept(requestBuilder);