Support o.s.h.HttpHeaders from both 6.x and 7.x Spring branches

[bedrin]

Replace invocation of org.springframework.web.reactive.function.server.ServerRequest.headers().asHttpHeaders().containsKey() with !org.springframework.web.reactive.function.server.ServerRequest.headers().header().isEmpty() to support Spring Framework 7.x

Motivation and Context

Spring Boot 4.0 and Spring Framework 7.0 release candidates are now available
Spring AI will use them as a baseline in version 2.0

There's a breaking change in Spring Framework 7.0 with few APIs removed from class org.springframework.http.HttpHeaders

One of removed methods is containsKey() which is used by modelcontextprotocol/java-sdk

This changed replaces it with !org.springframework.web.reactive.function.server.ServerRequest.headers().header().isEmpty() which would work both with Spring Framework 6.x and 7.x and thus enable the migration and early adoption of new framework versions.

How Has This Been Tested?

all tests are green

Breaking Changes

no breaking changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

N/A

[tzolov]

Thanks @bedrin