refactor: extract MCP echo server to fixtures file
Move the 113-line inline Python script to a proper file at
tools/server/tests/fixtures/mcp_echo_server.py
Benefits:
- Syntax highlighting and IDE support
- Can be linted/type-checked independently
- Serves as documentation for MCP server implementation
- Cleaner test code with simpler fixtures
- More visible in git history
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
