fix(vlm): serialize Transformers predictor execution

[qdivan]

Motivation

MinerU shares one predictor per backend/model tuple, while mineru-api accepts concurrent requests. For the Transformers backend, overlapping model.generate() calls can overwrite Qwen2-VL's request-specific rope_deltas state. This causes the empty-delta shape failures reported in #4376 and #4906; reducing global API concurrency to one avoids the failure but also serializes unrelated document work.

Modification

• Reuse MinerU's existing per-predictor execution guard for the Transformers backend, as is already done for MLX.
• Detect both explicit backend selection and the concrete Transformers client module.
• Keep vLLM, LMDeploy, HTTP, and async-vLLM predictors unlocked so their native concurrency is unchanged.
• Add focused sync/async regression coverage for Transformers serialization, MLX preservation, lock reuse, and unlocked concurrent backends.

This serializes only calls into a shared local Transformers predictor. PDF rendering and post-processing remain concurrent.

Verification

On upstream master at 79d6d8d79fb8f3ddba5cc34c07a16f0ec36f56c7:

• Base behavior: the focused regression had 2 failures and 1 error because Transformers predictors had no execution lock.
• Patched behavior: python3 tests/unittest/test_vlm_predictor_execution_guard.py — 5 passed (repeated three times during independent audit).
• python3 -m py_compile mineru/backend/vlm/vlm_analyze.py tests/unittest/test_vlm_predictor_execution_guard.py
• git diff --check
• Independent read-only audit: PASS.

No GPU/model run is required for this integration-level locking change: the regression directly exercises the production sync and async guard implementations without model downloads.

BC-breaking

No. Public APIs and output formats are unchanged. Only concurrent execution through a shared local Transformers predictor is serialized.

Checklist

• The reported failure is covered by focused regression tests.
• MLX behavior and concurrent vLLM/LMDeploy/HTTP behavior are covered.
• Static checks and focused tests pass.
• No documentation change is required for this internal concurrency fix.

[github-actions]

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[qdivan]

I have read the CLA Document and I hereby sign the CLA

[qdivan]

This is ready for maintainer review. The focused concurrency regression passes (5/5 across three consecutive runs), the CLA check is green, and the current head remains clean and mergeable. The patch narrowly serializes only the shared Transformers predictor path while preserving concurrency for other backends. Could a maintainer review it when available?