feat: add Spring Boot auto-configuration support to core SDK

[stenalpjolly]

Summary

This PR adds Spring Boot auto-configuration support to the core Java SDK, enabling developers to easily bootstrap new Spring Boot applications integrated with the MCP Toolbox.

Expectation & Implementation

• Instead of introducing a separate module or restructuring the monorepo directory tree, the auto-configuration has been embedded directly inside the mcp-toolbox-sdk-java module using <optional>true</optional> Spring dependencies.
• Added com.google.cloud.mcp.autoconfigure.McpToolboxProperties to declare configurable properties under the prefix google.cloud.mcp.toolbox.
• Added com.google.cloud.mcp.autoconfigure.McpToolboxAutoConfiguration to register the McpToolboxClient bean conditionally when the google.cloud.mcp.toolbox.base-url property is supplied.
• Registered the auto-configuration class in META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports.
• Added a complete sample Spring Boot 3.x application under example-spring-boot showing how developers can configure and autowire the client bean.

Test cases

• Created com.google.cloud.mcp.autoconfigure.McpToolboxAutoConfigurationTest validating bean loading, fallback logic, and custom bean override behavior.
• Verified compile compatibility of the new example-spring-boot application.
• All checks and tests passed locally under JDK 23 (compile, test, package).

Acceptance criteria

• Auto-configuration works out-of-the-box when the library is imported into a Spring Boot 3.x project.
• Non-Spring applications can import the library without any transitive Spring dependency pollution.

Breaking changes

None.