refactor: split MessageDict by role for mypy narrowing (#126) #278

Job	Run time
ruff + pip-audit (windows-latest)	1m 5s
Verify prod requirements.txt is sufficient	7s
pytest (ubuntu-latest)	47s
API integration tests + coverage (macos-latest)	28s
mypy	24s
API integration tests + coverage (ubuntu-latest)	18s
pytest (windows-latest)	1m 25s
Frontend unit tests (vitest) (windows-latest)	1m 0s
Performance benchmarks (gated)	31s
ruff + pip-audit (macos-latest)	26s
API integration tests + coverage (windows-latest)	50s
ruff + pip-audit (ubuntu-latest)	24s
Frontend unit tests (vitest) (macos-latest)	38s
pytest (macos-latest)	53s
Frontend unit tests (vitest) (ubuntu-latest)	27s
	9m 43s

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