Support non-String URI variable parameters for @McpResource methods

[OneZero-Y]

Fixes #6617

Summary

@McpResource methods with URI template variables currently require those parameters to be String. Declaring a primitive or wrapper type (e.g. int id) fails at startup with:

URI variable parameters must be of type String

This forces users to manually parse URI variables, which is inconsistent with Spring MVC-style path variable handling.

This PR removes the String-only restriction and converts URI variable values to the declared parameter type using Spring's DefaultConversionService. Invalid values are surfaced as McpError(INVALID_PARAMS) via the existing resource callback error handling.

Changes

• Add McpUriVariableValueConverter to convert URI variable strings to the declared parameter type
• Remove the String-only validation in AbstractMcpResourceMethodCallback
• Convert URI variable values when building method arguments in buildArgsWithUriVariables
• Add unit tests for successful int conversion and invalid value handling

Usage

Before (manual parsing required):

@McpResource(uri = "example.com/users/{id}")
public ResourceContents users(String id) {
    int userId = Integer.parseInt(id);
    // ...
}

After:

@McpResource(uri = "example.com/users/{id}")
public ResourceContents users(int id) {
    // id is already converted
}

Test plan

• ./mvnw -pl mcp/mcp-annotations test -Dtest=SyncMcpResourceMethodCallbackTests#testCallbackWithIntegerUriVariable,SyncMcpResourceMethodCallbackTests#testCallbackWithInvalidIntegerUriVariable,SyncMcpResourceMethodCallbackTests#testCallbackWithUriVariables -Dsurefire.failIfNoSpecifiedTests=false
• @McpResource with int URI variable parameter registers successfully at startup
• Valid URI value (42) is converted and passed to the handler method
• Invalid URI value (not-a-number) returns McpError
• Existing String URI variable behavior unchanged

[OneZero-Y]

Hey @dimitarproynov — would you mind taking a look when you get a chance? This lets @McpResource URI variables bind to non-String parameter types (e.g. int) via DefaultConversionService, instead of failing at startup. Invalid values surface as McpError(INVALID_PARAMS).