Added type hints to constant_concentration.py

[JC230903]

Description

This is a trial PR to test adding type hints. I’ve verified that it doesn’t change any mypy behavior at this stage, but it’s intended to gather feedback on approach and clarity. Happy to change this to a draft if needed.
Fixes #4954

Type of change

This PR focuses on:

Adding type hints to helper functions that return basic types.

Adding hints to class methods with simple input/output signatures.

Ensuring compatibility with existing code and mypy.

Motivation

I’d like to familiarize myself with PyBaMM’s contribution process and get early feedback on:

Type hinting conventions

Review expectations

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.70%. Comparing base (5b8cda0) to head (4c1e383).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ls/electrolyte_diffusion/constant_concentration.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4956      +/-   ##
===========================================
- Coverage    98.70%   98.70%   -0.01%     
===========================================
  Files          304      304              
  Lines        23701    23704       +3     
===========================================
+ Hits         23394    23396       +2     
- Misses         307      308       +1     

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