fix: resolve socket hang up errors for large file execution

[usnavy13]

Description

Fixes socket hang up errors during large-file or slow /exec requests by keeping the HTTP connection alive and removing blocking large-file reads from the asyncio event loop.

This PR includes the dev -> main delta at 2529fe6 over 0dc767a:

• stream keepalive whitespace from POST /exec while execution is still running
• stream large MinIO-backed files directly to disk before mounting into the sandbox
• add a functional regression test for concurrent requests during 50 MB file-backed execution

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Verified locally on March 11, 2026.

• pytest tests/unit/
  Result: 371 passed in 0.73s
• API_BASE=http://localhost:8080 API_KEY=<repo .env key> pytest tests/functional/test_concurrent_file_exec.py -v
  Result: 1 passed in 5.55s
• Local stack health check at http://localhost:8080/health

Notes:

• Functional tests default to http://localhost:8000, but this local Docker Compose stack exposes the API on http://localhost:8080, so the regression test was run against 8080.
• GitHub PR CI currently runs pytest tests/unit/ only; the new functional regression coverage is included here as local verification and should be treated as follow-up CI work.

Reviewer Notes

• POST /exec remains JSON-compatible, but responses may now begin with keepalive whitespace before the final JSON payload.
• The new stream path re-raises validation and service-unavailable errors for normal FastAPI handling; reviewers should pay attention to how unexpected runtime failures surface from the streaming response.
• No request or response schema fields changed.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules