feat: sep-1036 add URL Mode Elicitation (#843)

closes #410 

## How Has This Been Tested?
integration and unit tests 

## Breaking Changes
none

## Types of changes
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/ClientConnection.kt

@@ -25,6 +25,7 @@ import io.modelcontextprotocol.kotlin.sdk.types.ResourceListChangedNotification
 import io.modelcontextprotocol.kotlin.sdk.types.ResourceUpdatedNotification
 import io.modelcontextprotocol.kotlin.sdk.types.ServerNotification
 import io.modelcontextprotocol.kotlin.sdk.types.ToolListChangedNotification
+import io.modelcontextprotocol.kotlin.sdk.types.supportsUrl
 
 private val logger = KotlinLogging.logger {}
 
@@ -238,6 +239,11 @@ internal class ClientConnectionImpl(private val session: ServerSession) : Client
 
     override suspend fun createElicitation(request: ElicitRequest, options: RequestOptions?): ElicitResult {
         val params = request.params
+        if (params is ElicitRequestURLParams) {
+            require(session.clientCapabilities?.elicitation.supportsUrl) {
+                "Client did not advertise elicitation.url capability; cannot send a URL-mode elicitation."
+            }
+        }
         logger.debug {
             when (params) {
                 is ElicitRequestFormParams ->
@@ -299,6 +305,9 @@ internal class ClientConnectionImpl(private val session: ServerSession) : Client
     }
 
     override suspend fun sendElicitationComplete(notification: ElicitationCompleteNotification) {
+        require(session.clientCapabilities?.elicitation.supportsUrl) {
+            "Client did not advertise elicitation.url capability; cannot send an elicitation completion notification."
+        }
         logger.debug { "Sending elicitation complete notification for: ${notification.params.elicitationId}" }
         notification(notification)
     }

kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/Server.kt

@@ -46,6 +46,7 @@ import io.modelcontextprotocol.kotlin.sdk.types.ToolAnnotations
 import io.modelcontextprotocol.kotlin.sdk.types.ToolExecution
 import io.modelcontextprotocol.kotlin.sdk.types.ToolSchema
 import io.modelcontextprotocol.kotlin.sdk.types.UnsubscribeRequest
+import io.modelcontextprotocol.kotlin.sdk.types.UrlElicitationRequiredException
 import io.modelcontextprotocol.kotlin.sdk.utils.MatchResult
 import io.modelcontextprotocol.kotlin.sdk.utils.PathSegmentTemplateMatcher
 import io.modelcontextprotocol.kotlin.sdk.utils.ResourceTemplateMatcher
@@ -651,6 +652,9 @@ public open class Server(
             }
         } catch (e: CancellationException) {
             throw e
+        } catch (e: UrlElicitationRequiredException) {
+            // Surface a required URL-mode elicitation as a JSON-RPC error (-32042)
+            throw e
         } catch (e: Exception) {
             logger.error(e) { "Error executing tool ${requestParams.name}" }
             CallToolResult(