fix: Add overflow protection to memory estimation

[yudduy]

Fixes integer overflow crashes in memory estimation and partition operations during large dataset processing.

Changes:

• Memory estimation: Add overflow guards, cap at usize::MAX/2
• FixedSizeList: Limit 1M elements, check infinity
• Shuffle: Use checked_mul(), fallback on overflow
• Partitioning: Enforce 0 < n ≤ 100K

Breaking Change: IntoPartitionsConfig now requires validated constructor:

• Before: IntoPartitionsConfig { num_partitions: 100 }
• After: IntoPartitionsConfig::new(100)?

Testing: 25 new overflow tests, all existing tests pass

Relates to: #4724

[greptile-apps[bot]]

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[yudduy]

100k and 1M are heuristics and happy to add doc comments / tighten FixedSizeList cap if you'd prefer