Add F.plot(...) for one-line PyVista field visualization

[Copilot]

Description

Summary

This PR adds a lightweight plotting API for species fields so users can generate PyVista visuals in one line, including multi-species and subdomain-aware cases.

• API surface
  • Added public F.plot(...) entrypoint (src/festim/plot.py, exported via src/festim/__init__.py).
• Core behavior
  • Supports single species and list input (auto subplot layout).
  • Supports subdomain=... for mixed-domain/discontinuous workflows.
  • Supports filename=... for screenshot output and show_edges=True shortcut.
  • Forwards extra mesh args via **kwargs to plotter.add_mesh(...).
• Robustness
  • Clear errors for invalid field types, missing post-processing solutions, and missing optional pyvista dependency.
• Tests
  • Added focused unit tests in test/test_plot.py covering API behavior, edge cases, and error paths.

Related Issues

Handled by automatic issue linking in the PR system.

Motivation and Context

Plotting fields with PyVista currently requires repetitive boilerplate (vtk_mesh, grid wiring, plotter setup). This change introduces a small wrapper to make field visualization immediate while preserving control through optional arguments.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 🔨 Code refactoring (no functional changes, no API changes)
• 📝 Documentation update
• ✅ Test update (adding missing tests or correcting existing tests)
• 🔧 Build/CI configuration change

Testing

• All existing tests pass locally (pytest)
• I have added new tests that prove my fix is effective or that my feature works

Code Quality Checklist

• My code follows the code style of this project (Ruff formatted: ruff format .)
• My code passes linting checks (ruff check .)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

Documentation

• I have updated the documentation accordingly (if applicable)
• I have added docstrings to new functions/classes following the project conventions

Breaking Changes

None.

Screenshots/Examples

import festim as F

H = F.Species("H")
D = F.Species("D")

F.plot(H)                              # default quick view
F.plot(H, subdomain=vol1)              # mixed-domain: specific subdomain
F.plot([H, D], show_edges=True)        # automatic subplots
F.plot(H, filename="concentration.png")  # save screenshot

Additional Notes

This PR keeps scope intentionally narrow: one public plotting helper plus focused tests around that API surface.

[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.68%. Comparing base (9e99915) to head (85a0e26).

Files with missing lines	Patch %	Lines
src/festim/plotting.py	89.47%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1176      +/-   ##
==========================================
- Coverage   94.76%   94.68%   -0.09%     
==========================================
  Files          46       47       +1     
  Lines        3534     3592      +58     
==========================================
+ Hits         3349     3401      +52     
- Misses        185      191       +6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[RemDelaporteMathurin]

@ck768 @jhdark @hhy2022 would one of you mind having a look at these modifications?