Add URL elicitation support (SEP-1036) (#993)

Add URL-type elicitation schema support allowing servers to request URL
input from users during tool execution. This enables out-of-band
interactions like payment processing or API key entry.

Breaking changes:

- `ElicitRequest` changed from a `record` to an `interface`.
- The original `ElicitRequest` record was renamed to `ElicitFormRequest`.
- `McpClient` builder `elicitation()` methods now accept `ElicitFormRequest` instead of `ElicitRequest`.

New APIs:

- `ElicitUrlRequest` record for URL-mode elicitation.
- `urlElicitation()` builder methods in `McpClient`.
- `elicitationCompleteConsumer()` and `elicitationCompleteConsumers()` builder methods in `McpClient`.
- `sendElicitationComplete()` methods in `McpAsyncServer` and `McpSyncServer`.
- `McpError.URL_ELICITATION_REQUIRED` and `McpSchema.ErrorCodes.URL_ELICITATION_REQUIRED`.
- `ElicitationCompleteNotification` record and `METHOD_NOTIFICATION_ELICITATION_COMPLETE` constant.

Co-authored-by: Daniel Garnier-Moiroux <git@garnier.wf>
Co-authored-by: Sainath Reddy Bobbala <bsnr@amazon.com>

docs/server.md

@@ -715,9 +715,7 @@ var tool = SyncToolSpecification.builder()
         }
 
         // Request user confirmation
-        ElicitRequest elicitRequest = ElicitRequest.builder()
-            .message("Do you want to proceed with this action?")
-            .requestedSchema(Map.of(
+        ElicitRequest elicitRequest = ElicitFormRequest.builder("Do you want to proceed with this action?", Map.of(
                 "type", "object",
                 "properties", Map.of("confirmed", Map.of("type", "boolean"))
             ))
@@ -739,6 +737,40 @@ var tool = SyncToolSpecification.builder()
     .build();
 ```
 
+To request out-of-band URL elicitation, such as a user authorizing an OAuth flow:
+
+```java
+var urlTool = SyncToolSpecification.builder()
+    .tool(Tool.builder()
+        .name("oauth-auth")
+        .description("Authenticates via OAuth")
+        .inputSchema(schema)
+        .build())
+    .callHandler((exchange, request) -> {
+        // Request URL elicitation from client
+        if (
+                exchange.getClientCapabilities().elicitation() != null 
+                && exchange.getClientCapabilities().elicitation().url() != null
+        ) {
+            ElicitRequest urlRequest = McpSchema.ElicitUrlRequest
+                    .builder("Please authenticate", "https://example.com/oauth", "oauth-123").build();
+            ElicitResult result = exchange.elicit(urlRequest);
+            // handle result.action == CANCELLED or DENIED
+            if (result.action() != ElicitResult.Action.ACCEPT) {
+                return CallToolResult.builder()
+                        .content(List.of(new McpSchema.TextContent("Authentication failed or cancelled")))
+                        .build();
+            }
+        }
+
+        // wait for user to visit the URL
+        return CallToolResult.builder()
+                .content(List.of(new McpSchema.TextContent("Authentication successful")))
+                .build();
+    })
+    .build();
+```
+
 ### Logging Support
 
 The server provides structured logging capabilities that allow sending log messages to clients with different severity levels.