refactor: split MessageDict by role for mypy narrowing #273

Job	Run time
Frontend unit tests (vitest) (windows-latest)	59s
ruff + pip-audit (macos-latest)	16s
API integration tests + coverage (ubuntu-latest)	30s
Verify prod requirements.txt is sufficient	6s
ruff + pip-audit (ubuntu-latest)	18s
ruff + pip-audit (windows-latest)	1m 2s
Frontend unit tests (vitest) (ubuntu-latest)	24s
mypy	18s
API integration tests + coverage (macos-latest)	24s
API integration tests + coverage (windows-latest)	1m 55s
Performance benchmarks (gated)	28s
Frontend unit tests (vitest) (macos-latest)	41s
pytest (macos-latest)	53s
pytest (ubuntu-latest)	1m 3s
pytest (windows-latest)	1m 14s
	10m 31s

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