fix(client): gate completion/complete on the completions capability (#846)

Gate `completion/complete` on the server's `completions` capability
instead of `prompts`.

closes #845

## Motivation and Context

A server that declares `completions` but not `prompts` is currently
rejected for completion. Details in #845.

## How Has This Been Tested?

New `ClientAssertCapabilityTest` cases; they fail on `main` and pass
with this change. `apiCheck` / `ktlintCheck` / `detekt` pass.

## Breaking Changes

None.

## Types of changes

- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] 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

Co-authored-by: Pavel Gorgulov <devcrocod@gmail.com>

kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/Client.kt

@@ -242,13 +242,18 @@ public open class Client(private val clientInfo: Implementation, options: Client
 
             Method.Defined.PromptsGet,
             Method.Defined.PromptsList,
-            Method.Defined.CompletionComplete,
             -> {
                 checkNotNull(serverCapabilities?.prompts) {
                     "Server does not support prompts (required for $method)"
                 }
             }
 
+            Method.Defined.CompletionComplete -> {
+                checkNotNull(serverCapabilities?.completions) {
+                    "Server does not support completions (required for $method)"
+                }
+            }
+
             Method.Defined.ResourcesList,
             Method.Defined.ResourcesTemplatesList,
             Method.Defined.ResourcesRead,
@@ -394,7 +399,7 @@ public open class Client(private val clientInfo: Implementation, options: Client
      * @param params The completion request parameters.
      * @param options Optional request options.
      * @return The completion result returned by the server, or `null` if none.
-     * @throws IllegalStateException If the server does not support prompts or completion.
+     * @throws IllegalStateException If the server does not support completions.
      */
     public suspend fun complete(params: CompleteRequest, options: RequestOptions? = null): CompleteResult =
         request(params, options)

kotlin-sdk-client/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/ClientAssertCapabilityTest.kt

@@ -84,6 +84,25 @@ class ClientAssertCapabilityTest {
         ex.message.orEmpty() shouldContain "Client does not support tasks"
     }
 
+    @Test
+    fun `CompletionComplete does not throw when server declared completions`() = runTest {
+        val client = newTestClient(
+            serverCapabilities = ServerCapabilities(completions = ServerCapabilities.Completions),
+        )
+        client.exposedAssertCapabilityForMethod(Method.Defined.CompletionComplete)
+    }
+
+    @Test
+    fun `CompletionComplete throws when server has no completions capability`() = runTest {
+        val client = newTestClient(
+            serverCapabilities = ServerCapabilities(prompts = ServerCapabilities.Prompts()),
+        )
+        val ex = assertFailsWith<IllegalStateException> {
+            client.exposedAssertCapabilityForMethod(Method.Defined.CompletionComplete)
+        }
+        ex.message.orEmpty() shouldContain "Server does not support completions"
+    }
+
     private suspend fun newTestClient(
         serverCapabilities: ServerCapabilities = ServerCapabilities(),
         clientCapabilities: ClientCapabilities = ClientCapabilities(),