refactor: shared session load path for session and export APIs #274

Job	Run time
Performance benchmarks (gated)	31s
Frontend unit tests (vitest) (windows-latest)	1m 23s
Frontend unit tests (vitest) (macos-latest)	40s
ruff + pip-audit (windows-latest)	40s
API integration tests + coverage (windows-latest)	1m 48s
Verify prod requirements.txt is sufficient	11s
pytest (ubuntu-latest)	38s
API integration tests + coverage (macos-latest)	23s
ruff + pip-audit (macos-latest)	23s
API integration tests + coverage (ubuntu-latest)	19s
Frontend unit tests (vitest) (ubuntu-latest)	30s
ruff + pip-audit (ubuntu-latest)	14s
mypy	25s
pytest (macos-latest)	35s
pytest (windows-latest)	1m 42s
	10m 22s

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