feat: narrow MessageDict.role to RoleLiteral #174

Job	Run time
mypy	23s
Verify prod requirements.txt is sufficient	12s
API integration tests + coverage (windows-latest)	48s
ruff + pip-audit (ubuntu-latest)	21s
Performance benchmarks (informational)	27s
ruff + pip-audit (windows-latest)	1m 10s
API integration tests + coverage (macos-latest)	28s
Frontend unit tests (vitest) (windows-latest)	47s
ruff + pip-audit (macos-latest)	23s
Frontend unit tests (vitest) (macos-latest)	30s
Frontend unit tests (vitest) (ubuntu-latest)	18s
pytest (windows-latest)	1m 11s
API integration tests + coverage (ubuntu-latest)	22s
pytest (ubuntu-latest)	37s
pytest (macos-latest)	27s
	8m 24s

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