test: refactor shared 2D mesh helper into pytest fixture

[swastim01]

Description

This PR refactors the reusable 2D mesh generation helper from tests/shared.py into a pytest fixture defined in tests/conftest.py.
The change removes repeated mesh construction boilerplate across multiple test files and aligns with recommended fixture usage in PyBaMM tests.

Several tests have been updated to consume the shared fixture directly, while preserving existing test semantics.

Context: This follows the discussion and suggestion in #5294 and #5295 regarding using fixtures for shared setup with substantial reuse.

Contributes to issue: #4502

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #)

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

[agriyakhetarpal]

Nice work, thanks, @swastim01! I've launched the workflows and we will be good to go with this once the tests pass. One more change: since get_mesh_for_testing_2d is no longer used, could you please remove it?

[agriyakhetarpal]

@swastim01 almost there, just one more import to remove – see failing tests!

[swastim01]

@swastim01 almost there, just one more import to remove – see failing tests!

Hey @agriyakhetarpal, I missed that earlier, thanks for catching it! I’ve removed the leftover import now.

[agriyakhetarpal]

All good now, sorry this took me so long to review @swastim01! Thank you! I'll merge when tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.03%. Comparing base (9307a75) to head (92b2a18).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5342      +/-   ##
==========================================
- Coverage   98.60%   98.03%   -0.57%     
==========================================
  Files         324      327       +3     
  Lines       28219    28690     +471     
==========================================
+ Hits        27824    28126     +302     
- Misses        395      564     +169     

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