fix(parallel_gripper): rename variables for consistency (backport #2314)

[mergify]

Description

Rename internal variables in parallel_gripper_controller for consistency
and clarity.

Relates to #1229

Is this user-facing behavior change?

Did you use Generative AI?

Yes, Claude (Anthropic) was used to help understand the codebase and
discuss the changes. The actual code changes were made manually.

Additional Information

This is the first of multiple PRs addressing the issues raised in #1229.
Each issue will be addressed in a separate PR as suggested by the maintainer.

---

This is an automatic backport of pull request #2314 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

❌ Patch coverage is 21.05263% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.91%. Comparing base (58fabc7) to head (cbf7feb).
⚠️ Report is 1 commits behind head on jazzy.

Files with missing lines	Patch %	Lines
...roller/parallel_gripper_action_controller_impl.hpp	21.05%	15 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            jazzy    #2315   +/-   ##
=======================================
  Coverage   84.91%   84.91%           
=======================================
  Files         154      154           
  Lines       15265    15265           
  Branches     1350     1350           
=======================================
  Hits        12963    12963           
  Misses       1811     1811           
  Partials      491      491           

Flag	Coverage Δ
unittests	84.91% <21.05%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...roller/parallel_gripper_action_controller_impl.hpp	25.25% <21.05%> (ø)

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