refactor: shared session load path for session and export APIs #275

Job	Run time
API integration tests + coverage (macos-latest)	23s
API integration tests + coverage (ubuntu-latest)	26s
Verify prod requirements.txt is sufficient	8s
Performance benchmarks (gated)	31s
Frontend unit tests (vitest) (windows-latest)	1m 17s
mypy	30s
Frontend unit tests (vitest) (macos-latest)	35s
Frontend unit tests (vitest) (ubuntu-latest)	29s
pytest (macos-latest)	37s
API integration tests + coverage (windows-latest)	54s
ruff + pip-audit (ubuntu-latest)	20s
pytest (windows-latest)	1m 27s
ruff + pip-audit (macos-latest)	16s
ruff + pip-audit (windows-latest)	49s
pytest (ubuntu-latest)	43s
	9m 25s

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