feat: narrow MessageDict.role to RoleLiteral #172

Job	Run time
Performance benchmarks (informational)	26s
pytest (ubuntu-latest)	40s
pytest (windows-latest)	1m 10s
pytest (macos-latest)	40s
mypy	19s
ruff + pip-audit (windows-latest)	1m 48s
Frontend unit tests (vitest) (macos-latest)	21s
Frontend unit tests (vitest) (ubuntu-latest)	15s
ruff + pip-audit (ubuntu-latest)	19s
API integration tests + coverage (windows-latest)	43s
ruff + pip-audit (macos-latest)	24s
Verify prod requirements.txt is sufficient	11s
Frontend unit tests (vitest) (windows-latest)	47s
API integration tests + coverage (macos-latest)	18s
API integration tests + coverage (ubuntu-latest)	21s
	8m 42s

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