feat: Improve Tool schema handling

- Make McpSchema final and add private constructor
- Add JsonSchema record for structured schema representation
- Change Tool.inputSchema type from Map<String, Object> to JsonSchema.
- Use the String JSON Schema constructor when creating the Tool programaticaly.
- Enhance StdioClientTransport error logging with more context
- Add ToolHelper utility class for:
  - Converting Spring AI FunctionCallbacks to MCP tools
  - Generating JSON schemas for tool input validation
  - Facilitating integration between Spring AI functions and MCP tools
- Update tests and samples to use the new Tool schema format

Resolves #52