docs(server): add unit testing guide using InMemoryTransport

[nielskaspers]

Summary

Closes #1126

There was no documentation explaining how to unit-test MCP servers without spawning subprocesses or opening ports. This adds a ## Testing section to the server guide that:

• Explains InMemoryTransport.createLinkedPair() as the recommended approach
• Shows a complete vitest/jest example for testing a registered tool end-to-end
• Lists what this approach does and does not test (so readers know when they need HTTP integration tests too)
• Links to the existing createInMemoryTaskEnvironment helper for task-related tests

Changes

• docs/server.md: new ## Testing section before ## More server features

Test plan

• Read through the new section — the code example should be copy-paste runnable in a project that depends on @modelcontextprotocol/server and @modelcontextprotocol/core

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