feat: narrow MessageDict.role to RoleLiteral #180

Job	Run time
ruff + pip-audit (macos-latest)	24s
Verify prod requirements.txt is sufficient	10s
ruff + pip-audit (windows-latest)	55s
Frontend unit tests (vitest) (windows-latest)	44s
mypy	20s
ruff + pip-audit (ubuntu-latest)	28s
API integration tests + coverage (windows-latest)	56s
pytest (ubuntu-latest)	54s
pytest (windows-latest)	1m 20s
Frontend unit tests (vitest) (ubuntu-latest)	19s
API integration tests + coverage (ubuntu-latest)	19s
API integration tests + coverage (macos-latest)	14s
Performance benchmarks (informational)	29s
Frontend unit tests (vitest) (macos-latest)	24s
pytest (macos-latest)	29s
	8m 25s

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