test: 97.8% coverage — 28 tests, all handler paths at 100%

Added 13 edge-case tests:
- Decode error, notification handling, invalid params (3 handlers)
- Handler errors (resources, prompts), unknown tool/resource/prompt
- Encoder write errors (initialize, default, tools/list,
  resources/list, prompts/list)

Only untested code: Run() one-line OS wrapper (os.Stdin/os.Stdout)

Author: molty3000