Fix everything-server-based tests

[chemicL]

Use a fixed npx @modelcontextprotocol/server-everything version.
Replace tzolov/mcp-everything-server Docker image with node:lts-alpine + npx, handle HTTP 202, fix test assertions

Motivation and Context

The recent rollout of everything-server broke our tests which take the latest version from the node registry.
The change unifies the everything-server usage - the Testcontainers Docker setup uses the same version as the STDIO npx-based tests and no longer relies on tzolov/mcp-everything-server.

Unfortunately, there are more problems with the latest version of everything-server (@modelcontextprotocol/[email protected]) so the one used for tests is 2025.12.18.

How Has This Been Tested?

Automatic test suite.

Breaking Changes

No.

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

No.