fix(protocol-engine): Fix hanging after cancellation

[SyntaxColoring]

Overview

Fixes RQA-2091

Test Plan

• Flex and OT-2 protocols can still succeed
• Flex protocols, when cancelled, home the X/Y/Z axes but leave the plungers and tip ejectors alone
• OT-2 protocols, when cancelled, eject the tips in the fixed trash if there are any tips attached

Changelog

• After a protocol ends, always call hardware_api.stop before doing anything further with the hardware. This is the bug fix.
• Formerly, if a protocol ended without any tips attached, we were taking the opportunity to home the plungers. That's dropped in this PR; for simplicity, we will never ever home the plungers as part of run cleanup. Homing the plungers is taken care of at run start. @sfoster1 and I agree that there was no benefit to homing the plungers during run cleanup, and that it could only open up opportunities for something to go wrong.

Review requests

There are opportunities for further cleanup here. For example, HardwareStopper._drop_tip() is also stopping and homing, which makes the overall sequence hard to follow and seems outside of its responsibility.

Since this PR has already been tested without that cleanup, let's not do it in this PR—but if you have any ideas, please do leave comments.

Risk assessment

Medium. I haven't looked at how well this code is covered by unit tests, but this PR is passing CI without having had to change anything, which suggests there are gaps.

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Co-authored with @jbleon95 and @sfoster1

[codecov]

Codecov Report

Merging #14215 (96d2e0f) into chore_release-7.1.0 (a7d1187) will decrease coverage by 0.01%.
Report is 1 commits behind head on chore_release-7.1.0.
The diff coverage is n/a.

Additional details and impacted files

@@                   Coverage Diff                   @@
##           chore_release-7.1.0   #14215      +/-   ##
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- Coverage                70.42%   70.42%   -0.01%     
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  Files                     2512     2512              
  Lines                    71296    71292       -4     
  Branches                  9006     9006              
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- Hits                     50212    50208       -4     
  Misses                   18879    18879              
  Partials                  2205     2205              

Flag	Coverage Δ
g-code-testing	96.44% <ø> (ø)
notify-server	89.13% <ø> (ø)

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Files	Coverage Δ
...rons/protocol_engine/execution/hardware_stopper.py	100.00% <ø> (ø)

[sfoster1]

Looks good to me if it works, I think this is the right approach for homing

[jbleon95]

Fixes the cancellation bug and looks correct to me