fix(api): trim glibc heap after document analysis to prevent OOM

[zuoanCo]

When running the MinerU API for a long time, RSS kept growing across documents because freed C heap pages were not returned to the OS.

• Add trim_process_heap() that calls malloc_trim(0) on glibc systems.
• Add MINERU_MALLOC_TRIM env var to control the behavior; defaults to enabled on Linux and disabled elsewhere.
• Extend clean_memory() to optionally invoke heap trimming.
• Call trim_process_heap() after every processing window in VLM and Hybrid sync/async doc_analyze.
• Call clean_memory() at the end of VLM doc_analyze paths.
• Add regression tests for the allocator hook and configuration.

Fixes #5313

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