fix handling different sse endpoints (relative/absolute)

[Fuud]

Motivation and Context

Currently absolute endpoints are handling incorrectly.
For example, IDE Index Plugin uses sse endpoint http://127.0.0.1:29170/index-mcp/sse and returns /index-mcp?sessionId=.... It should result in POST endpoint http://127.0.0.1:29170/index-mcp?sessionId=... but currently resolves in http://127.0.0.1:29170/index-mcp/index-mcp?sessionId=..

You can check TS implementation:
https://github.com/modelcontextprotocol/typescript-sdk/blob/main/packages/client/src/client/sse.ts#L181
Or java implementation:
https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java#L337
https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/util/Utils.java#L71

How Has This Been Tested?

I built project and tested it against IDE Index Plugin.
I tried to run tests but:

1. tests in kotlin-sdk-test/src/commonTest are ignored (and incorrent: it expectes that sse server is echo server that is incorrect. check)
2. tests in kotlin-sdk-test/src/jvmTest are passed except npm-related.

Breaking Changes

No (except if users uses incorrect written server)

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

Feel free advice me what tests should be added.

[Fuud]

@kpavlov Hi. Could you please review it.

[kpavlov]

We’re still looking for ways to automate our tests.

[kpavlov]

We need an automated test verifying the updated behavior