Add MCPTool/MCPToolset lazy init

[vblagoje]

Why

• Introduces optional lazy connect so pipelines can validate without triggering connect.
• Preserves current eager behavior by default (no runtime behavior change unless opted in).
• fixes MCPToolset raises MCPConnectionError in init if authentication fails haystack#9807

What

• Added eager_connect: bool = True to MCPTool and MCPToolset
• MCPTool: added _ensure_connected(); in lazy mode uses a permissive schema then tightens after first invoke.
• MCPToolset: if eager_connect=False, returns an empty toolset at construction; eager discovery path unchanged.
• Added test_mcp_tool_lazy_connect_updates_schema unit test

How Can It Be Used

• Lazy single tool (connects on first use):
  • tool = MCPTool(name="add", server_info=InMemoryServerInfo(...), eager_connect=False)
  • result = json.loads(tool.invoke(a=2, b=5))
• Eager (default) single tool:
  • tool = MCPTool(name="add", server_info=InMemoryServerInfo(...)) # unchanged behavior

How Did You Test It

• Added unit test validating lazy construction, first-use connect.
• Ran existing unit/integration tests to ensure no regressions.
• Ran itinerary agent with bunch of MCP tools to see if there are any regressions - none!

Notes For The Reviewer

• Default behavior is unchanged; lazy path only activates with eager_connect=False.
• Serialization now includes eager_connect; deserialization defaults to True if missing (backward compatible).
• Changes are surgical, confined to mcp_tool.py, mcp_toolset.py, and one new test.
• Lazy connect is thread-safe (RLock)

[vblagoje]

@Amnah199 converting to draft - getting some unusual behaviour with eager_connect = False in itinerary agent