Bundle BDN Hot Load client in Truss images

[angelamontemayor]

Summary

• bundle the dependency-free bdn_hotload Python client into standard Truss inference images
• expose mount CRUD, readiness polling, idempotent create retries, and typed errors over the pod-local Unix socket
• verify the package is imported from the emulated built image at /app

This adds the image-side client for BDN Hot Load. It intentionally does not add NIXL integration. Custom docker_server images are not included because they do not copy the standard Truss server template.

Validation

• uv run ruff check truss/templates/server/bdn_hotload truss/tests/templates/server/test_bdn_hotload.py
• uv run ruff format --check truss/templates/server/bdn_hotload truss/tests/templates/server/test_bdn_hotload.py
• uv run pytest truss/tests/templates/server/test_bdn_hotload.py -q (6 passed)
• Python 3.9 compileall for truss/templates/server/bdn_hotload

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