refactor: split MessageDict by role for mypy narrowing #277

Job	Run time
ruff + pip-audit (windows-latest)	1m 36s
mypy	22s
API integration tests + coverage (windows-latest)	53s
API integration tests + coverage (macos-latest)	23s
Frontend unit tests (vitest) (ubuntu-latest)	26s
Performance benchmarks (gated)	31s
API integration tests + coverage (ubuntu-latest)	18s
ruff + pip-audit (macos-latest)	30s
Verify prod requirements.txt is sufficient	7s
pytest (ubuntu-latest)	54s
Frontend unit tests (vitest) (macos-latest)	27s
ruff + pip-audit (ubuntu-latest)	29s
pytest (windows-latest)	1m 49s
Frontend unit tests (vitest) (windows-latest)	56s
pytest (macos-latest)	32s
	10m 13s

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