[release/v1] Fix integer underflow in BITCOUNT ... BIT byte-boundary mask (backport #1994)

[vazois]

Backport of #1994 to release/v1.

Summary

Fixes an integer underflow in BITCOUNT ... BIT (BitmapManagerBitCount.cs) that returned non-zero counts for unset bits whenever a BIT-mode range ended on a byte boundary (endOffset % 8 == 7).

The inclusive right bit index was computed as (int)((endOffset + 1) & 7), which wraps to 0 at byte boundaries and produces a wrapping byte-mask underflow. Fixed to use the same formula already used correctly in BitmapManagerBitPos.cs:

int rightBitIndex = (int)(endOffset & 7) + 1;

Tests

• Added BitmapBitCountBitBoundaryUnderflowTest (byte 0x80) covering single-bit and boundary-crossing BIT ranges.
• Updated BitmapBitCountLongBitOffsetParsingTest: its previous expectation of 8 was an artifact of the underflow. Under Garnet''s modulo-wrapping negative-offset semantics the clamped start wraps to bit index 1, so bits 1..7 of 0xFF = 7.
• Verified both tests pass on release/v1 (net10.0). The NormalizeBitCountOffsets / MaxOffsetForBitmapLength / ProcessNegativeOffset semantics are identical on this branch.

Cherry-picked from squash-merge 52aef6e (#1994 on main).

[Copilot]

Pull request overview

This PR backports the fix for a BITCOUNT ... BIT edge case where ranges ending exactly on a byte boundary could produce a wrapping mask and incorrectly count unset bits as set.

Changes:

• Fix BitmapManagerBitCount.BitIndexCount(byte* ...) to compute the right (exclusive) bit index as (endOffset & 7) + 1, preventing byte-boundary wrap to 0.
• Add a regression test for the byte-boundary underflow scenario (including single-bit and boundary-crossing ranges).
• Update an existing long-negative-offset parsing test expectation to reflect correct modulo-wrapping semantics after the fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/Garnet.test/GarnetBitmapTests.cs	Adds a regression test for the byte-boundary BIT underflow and updates an existing assertion impacted by the corrected mask logic.
libs/server/Resp/Bitmap/BitmapManagerBitCount.cs	Fixes right-bit-index calculation to avoid byte-boundary wraparound and mask underflow in BIT-mode counting.

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