[python] Preserve full precision in decimal global index keys - #10363

Purpose

DECIMAL global-index keys currently depend on Python's ambient decimal context. A valid DECIMAL(38,2) value such as 123456789012345678901234567890123456.78 fails to serialize at the default precision of 28, and decoding its existing unscaled bytes silently rounds away digits. This can break BTree/bitmap equality and range lookups.

Use a dedicated context with sufficient precision for scale rounding and reconstruct decoded decimals exactly from their coefficient and scale. Preserve HALF_UP rounding and the existing compact/non-compact byte formats without changing the caller's context.

Tests

• New regression coverage for compact and 38-digit decimals, negative values, scale 38, zero, exact decoding, rounding with caller traps enabled, and persisted BTree/bitmap equality and range queries.
• All 11 new tests pass; 10 fail against the unchanged base.
• Existing global_index_build_test.py and global_index_scalar_search_mode_test.py pass (44 tests).
• Flake8 for changed files and git diff --check pass.