refactor(mcp): consolidate MCP server auto-configuration modules and introduce protocol-based configuration

[tzolov]

• Consolidate multiple MCP server modules into unified webmvc/webflux modules
• Move properties classes from separate module into mcp-server-common
• Merge stateless auto-configuration into common module
• Replace transport-specific modules with protocol-based configuration
• Add ServerProtocol enum (SSE, STREAMABLE, STATELESS) for protocol selection
• Rename ServerType to ApiType for clarity
• Update conditional configuration to use protocol-based conditions
• Remove separate streamable/stateless starter modules
• Update documentation to reflect unified protocol-based approach
• Simplify module structure while maintaining backward compatibility through configuration

This refactoring reduces complexity by consolidating 7+ modules into 3 main modules (common, webmvc, webflux) while providing the same functionality through the spring.ai.mcp.server.protocol configuration property.

[tzolov]

Rebased and merged at e71da4e

[Fottas]

Hi @tzolov,
I've created PR #4218 that consolidates the configuration into a single unified structure to avoid the "Duplicated prefix" warnings. This approach draws inspiration from Spring's patterns like CacheProperties. Do you think this approach is suitable?

[Fottas]

Hi @tzolov,
I noticed PR #4218 had build failures due to NullPointerException in McpToolUtils, so I fixed that issue.
I see another fix was submitted later for the same NPE, but checking the MCP SDK source code, it's still using the deprecated call() parameter in AsyncToolSpecification. The runtime actually only uses callHandler() (see McpAsyncServer.java).
While working on that, I noticed PR #4218 is still pending - it addresses both the NPE fix with correct API usage and the "Duplicated prefix" warnings we discussed. Would you like me to rebase it on the latest changes?