fix(python): use mimalloc as global allocator to fix RSS leak

[mansiverma897993]

Resolves #7242

This PR configures mimalloc as the global allocator in the Python library (pylance) to resolve an unbounded RSS memory leak caused by glibc per-thread arena fragmentation.

Root Cause

Under glibc, sequential merge_insert workloads on Tokio worker threads allocate temporary buffers. Glibc does not eagerly return pages from these per-thread arenas to the OS after they are freed, causing RSS memory to grow monotonically until the process is terminated by the OOM killer.

Solution

• Added mimalloc dependency to python/Cargo.toml (with default-features = false to keep dependencies minimal).
• Registered mimalloc::MiMalloc as the #[global_allocator] in python/src/lib.rs.

[github-actions]

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