feat: Session ID ASCII validation check

[nbarbettini]

Closes #197

Motivation and Context

Closes a small but meaningful gap in spec coverage (a MUST).

How Has This Been Tested?

New unit tests.

Ran against modelcontextprotocol/typescript-sdk:

  node dist/index.js server --url http://localhost:3100/mcp --scenario server-initialize

  Checks:
  [server-initialize              ] SUCCESS Server responds to initialize request with valid structure
  [server-session-id-visible-ascii] SUCCESS Server-provided session ID uses only visible ASCII characters

  Test Results:
  Passed: 2/2, 0 failed, 0 warnings

Breaking Changes

None

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

[pkg-pr-new]

Open in StackBlitz

npx https://pkg.pr.new/@modelcontextprotocol/conformance@206

commit: d355b2d