`OpentronsOT2Backend`: support the 8-channel pipette#1135

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BioCam:ot2-head8-channels

Commits on Jul 8, 2026

`OpentronsOT2Backend`: model a multi mount as 8 channels (head8 step 1)

BioCam
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claude
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`OpentronsOT2Backend`: one command per column for multi ops (head8 step 2)

BioCam
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claude
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Audit OT-2 tests: drop redundant/weak cases and tautological asserts

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claude
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