Add mcp-client module: ChatClient calling tools from real external MCP servers over stdio

defaultToolCallbacks(ToolCallbackProvider) wires two real stdio MCP servers (the official
filesystem server and git server) into ChatClient, contrasted with defaultTools(Object) for a
local @Tool method. Every call -- MCP-sourced or local -- is logged through one Micrometer
ObservationHandler<ToolCallingObservationContext>; a first version wired that handler two ways
at once and every call logged twice, which is now a regression test. No real LLM is used
anywhere: every test builds an AssistantMessage.ToolCall by hand and drives it through the real
ToolCallingManager bean against real npx/uvx-launched MCP server processes.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01FtpJvZfg4nvLvtzgJTDWpB

mcp-client/src/main/resources/application.yml

@@ -0,0 +1,31 @@
+spring:
+  application:
+    name: mcp-client
+  ai:
+    # No real model call is made anywhere in this module's tests (see ToolCallingManagerTest) --
+    # this starter exists only to give the demo a ChatClient.Builder to wire tool callbacks into.
+    openai:
+      api-key: ${OPENAI_API_KEY:not-a-real-key-see-readme}
+    mcp:
+      client:
+        enabled: true
+        name: mcp-client-demo
+        version: 1.0.0
+        request-timeout: 20s
+        type: SYNC
+        toolcallback:
+          enabled: true
+        stdio:
+          connections:
+            filesystem:
+              command: ${MCP_CLIENT_NPX_CMD:npx}
+              args:
+                - "-y"
+                - "@modelcontextprotocol/server-filesystem"
+                - ${MCP_CLIENT_FS_ROOT:./fixtures/workspace}
+            git:
+              command: ${MCP_CLIENT_UVX_CMD:uvx}
+              args:
+                - "mcp-server-git"
+                - "--repository"
+                - ${MCP_CLIENT_GIT_REPO:./fixtures/demo-repo}