refactor: split MessageDict by role for mypy narrowing #276

Job	Run time
Verify prod requirements.txt is sufficient	10s
API integration tests + coverage (ubuntu-latest)	21s
Performance benchmarks (gated)	31s
pytest (macos-latest)	51s
Frontend unit tests (vitest) (ubuntu-latest)	27s
mypy	22s
ruff + pip-audit (ubuntu-latest)	20s
Frontend unit tests (vitest) (windows-latest)	1m 1s
ruff + pip-audit (macos-latest)	34s
pytest (ubuntu-latest)	46s
API integration tests + coverage (windows-latest)	1m 5s
Frontend unit tests (vitest) (macos-latest)	43s
API integration tests + coverage (macos-latest)	17s
ruff + pip-audit (windows-latest)	1m 2s
pytest (windows-latest)	1m 31s
	10m 1s

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