[Bug] `RGB_MATRIX_SPLIT` + WS2812 behaves wrong on right side

Describe the Bug

As diagnosed a few weeks ago (forgot to open this before) at discord

Culprit seems to be that #24364 removed/broke the right-side check for "if index is not within this half's boundaries, do nothing".

This is, with "split_count": [X, Y]:

• Using an index [0, X) on left side should affect those LEDs: works
• Using an index [X, X+Y) on right side should affect those LEDs: works
• Using an index [X, X+Y) on left side should be ignored: works
• Using an index [0, X) on right side should be ignored: doesn't work, it affects leds [X, X+X) AKA first X leds on the half's chain

Keyboard Used

No response

Link to product page (if applicable)

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Operating System

No response

qmk doctor Output

Is AutoHotKey / Karabiner installed

• AutoHotKey (Windows)
• Karabiner (macOS)

Other keyboard-related software installed

No response

Additional Context

No response