## Problem MCP prompt argument completion appears covered by the functional test suite, but the current test bypasses the MCP JSON-RPC completion path. `McpServerFunctionalTest::assertQueryCompletionProvider()` directly instantiates the `test_query` Drupal completion provider plugin and calls `getCompletions()`, so it does not exercise prompt registration, `McpServerFactory::buildCompletionProviders()`, SDK provider lookup, the `completion/complete` request handler, or the JSON-RPC response shape. The `test_with_args` prompt fixture also defines arguments without any `completion_providers`, so a real client completion request for those arguments would receive an empty completion result. ## Expected Behavior The functional test fixture should attach a completion provider to a prompt argument, and the functional test should send a real MCP `completion/complete` request against that prompt and argument. The test should assert the JSON-RPC response contains the expected completion values under `result.completion.values`, proving the Drupal provider is wired into the SDK and reachable by MCP clients. ## Steps to Reproduce 1. Inspect `tests/src/Functional/McpServerFunctionalTest.php` and note that `assertQueryCompletionProvider()` calls the plugin manager directly instead of using `callMcp('completion/complete', ...)`. 2. Inspect `tests/modules/mcp_server_test/config/install/mcp_server.mcp_prompt_config.test_with_args.yml` and note that neither `skill_level` nor `context` has a `completion_providers` entry. 3. Run `cd /var/www/html && vendor/bin/phpunit --filter McpServerFunctionalTest web/modules/contrib/mcp_server/`; the test passes despite not exercising MCP prompt argument completion end to end. ## Additional Context Relevant code path: `McpServerFactory::registerPrompt()` passes `buildCompletionProviders($raw_arguments)` as the third argument to `Builder::add()`. `buildCompletionProviders()` skips arguments without `completion_providers`, and the SDK's `CompletionCompleteHandler` returns an empty completion result when no provider is registered for the requested argument name.