feat: narrow MessageDict.role to RoleLiteral (#89) #181

Job	Run time
pytest (windows-latest)	1m 9s
Performance benchmarks (informational)	27s
pytest (macos-latest)	34s
Frontend unit tests (vitest) (macos-latest)	24s
Verify prod requirements.txt is sufficient	10s
Frontend unit tests (vitest) (ubuntu-latest)	20s
ruff + pip-audit (windows-latest)	1m 13s
API integration tests + coverage (windows-latest)	1m 0s
Frontend unit tests (vitest) (windows-latest)	1m 46s
API integration tests + coverage (macos-latest)	18s
ruff + pip-audit (ubuntu-latest)	21s
API integration tests + coverage (ubuntu-latest)	17s
mypy	20s
pytest (ubuntu-latest)	48s
ruff + pip-audit (macos-latest)	24s
	9m 31s

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