Enable .probe_liquid_height() to Detect Absence of Liquid

[BioCam]

Problem

Currently STARBackend.probe_liquid_heights() will error out if no liquid is detected instead of correctly detecting the absence of liquid (i.e. liquid_height=0.0 mm).
Consequently, STARBackend.probe_liquid_volumes() is also broken, and dispensing with cLLD into volume-non-deterministic destination containers is a hazard that can break aPs.

Solution / This PR

STARBackend.probe_liquid_heights() already calculates the lowest immersion position to be the well cavity bottom for safety. If a container is empty this PR introduces the catching of the specific firmware error and converts it into liquid_height=0.0mm.
At the same time, a warning is logged for these containers since it is possible to have edge cases (e.g. if cLLD: low-volume liquids don't have enough dissipation volume to be detected; ultra-pure liquids are non-conductive)

[Copilot]

Pull request overview

This PR enhances the .probe_liquid_height() method to gracefully handle the absence of liquid in containers, rather than raising unhandled firmware errors. When liquid is not detected, the method now returns a height of 0.0 (relative to the well bottom) and logs a warning with helpful diagnostic information.

Changes:

• Modified asyncio.gather() calls to capture and handle exceptions using return_exceptions=True
• Added error handling logic to distinguish "no liquid found" firmware errors from other exceptions
• Implemented consistency checking across replicates to detect edge cases where liquid detection is intermittent
• Updated height calculation to use cavity bottom as absolute height when no liquid is detected (resulting in 0.0 relative height)

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