Fix condor runner after stage budget removal

[twybo]

Fix condor_runner.py after the shared stage-out budget was removed. The runner still imported and called the deleted begin_stage_budget, which prevented it from starting.

This change:

• Removes the stale import and invocation.
• Preserves five independent stage-out attempts for each output.
• Adds regression tests for runner imports, retry behavior, stage-out failures, and fatal LFN preflight failures.
• Updates the validation guide to match the current GFAL and filesystem publication behavior.

Verification

• tests/test_stage_out.py: 33 passed
• tests/test_merge_planning.py: 27 passed
• Full test suite: 60 passed
• condor_runner import smoke test: passed