[Bugfix][Core] Reject non-positive output processing chunk sizes

[git-jxj]

Purpose

VLLM_V1_OUTPUT_PROC_CHUNK_SIZE controls how many engine outputs the V1
async output handler processes before yielding to the event loop. The value is
used as the step argument to range() without validation:

• 0 raises ValueError and stops the output handler.
• A negative value produces an empty range, so pending outputs are silently
  skipped.

Validate the value when the environment variable is read so invalid
configurations fail early with a clear error. The default value (128) and all
positive overrides keep their existing behavior.

Duplicate check: no matching issue or open PR was found after searching for
VLLM_V1_OUTPUT_PROC_CHUNK_SIZE, output processing chunk size, and
output_proc_chunk_size in vllm-project/vllm.

Changes

• Parse and validate VLLM_V1_OUTPUT_PROC_CHUNK_SIZE in a dedicated getter.
• Reject zero and negative values with a descriptive ValueError.
• Cover the default, a positive override, zero, and a negative value in the
  existing environment-variable test suite.

Test Plan

.venv/bin/python -m pytest tests/test_envs.py -v
.venv/bin/pre-commit run --files vllm/envs.py tests/test_envs.py

Test Result

Before the fix, the new non-positive-value regression cases fail because both
0 and -1 are accepted. Downstream, zero crashes range() while a negative
step skips the output-processing loop.

After the fix:

59 passed, 16 warnings in 15.60s
All applicable pre-commit hooks passed.

Model evaluation: not applicable. This change only rejects invalid
configuration values and does not change model outputs, accuracy, or valid
serving behavior.

AI Assistance

OpenAI Codex assisted with this change.

[claude]

Claude Code Review

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