Add workflow export bundle API endpoint with artifact existence flags

[akgohain]

Motivation

• Provide a backend endpoint to export a reproducible workflow bundle containing session snapshot, chronological events, and referenced artifact pointers.
• Ensure the export is deterministic and self-describing with a schema version and timestamp.
• Represent missing artifact files safely (boolean exists) instead of hard-failing.
• Reuse existing workflow DB models/serializers and avoid frontend changes.

Description

• Add Pydantic response models in server_api/workflow/models.py for WorkflowExportBundle, WorkflowSessionSnapshot, WorkflowEvent, and ArtifactReference.
• Implement POST /api/workflows/{workflow_id}/export-bundle in server_api/workflow/router.py to load EHToolSession and EHToolLayer, build ordered events, deduplicate artifact paths, and return exists flags using os.path.exists.
• Events are timestamped and sorted so returned events are chronological and deterministic.
• Register the new workflow router in server_api/main.py and add unit tests in tests/test_workflow_export_bundle.py covering the happy path and missing-artifact behavior.

Testing

• Ran uv run pytest -q tests/test_workflow_export_bundle.py which failed in this environment due to editable package metadata resolution for pytorch_connectomics.
• Ran PYTHONPATH=. uv run --no-project pytest -q tests/test_workflow_export_bundle.py which passed (2 passed).

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Codex Task

[akgohain]

Consolidated into draft PR #165 (feat/codex-wave-integration). Closing this source draft to keep review in one place.