Deprecate pybamm.Magnitude in favour of Component

[aabills]

Summary

• Deprecate pybamm.Magnitude with a DeprecationWarning pointing users to pybamm.Component(symbol, 0|1).
• Migrate internal callers (BasicDFN2D, 2D FV / processed-variable tests) to Component.
• Teach FiniteVolume2D edge-averaging to accept Component indices so the replacement works for flux products.

Stack

1. Generalise VectorField to N components #5686 — VectorField N-comp
2. Add unstructured mesh infrastructure #5687 — Meshing
3. Add unstructured finite volume spatial method #5688 — Spatial method + ProcessedVariable
4. Add VTK plotting for unstructured meshes #5689 — VTK plotting
5. Add unstructured 2D/3D DFN battery models #5690 — Unstructured DFN models
6. This PR (Deprecate pybamm.Magnitude in favour of Component #5691) — Deprecate Magnitude

Test plan

• test_vector_field_and_magnitude.py (Component + deprecation)
• test_process_binary_operators, test_processed_variable_2D_fvm, 2D integration tests
• CI unit suite

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.84%. Comparing base (5e1d810) to head (47e87fd).

Files with missing lines	Patch %	Lines
...amm/src/pybamm/spatial_methods/finite_volume_2d.py	70.00%	3 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           ufv-4-models    #5691   +/-   ##
=============================================
  Coverage         96.84%   96.84%           
=============================================
  Files               344      344           
  Lines             34489    34499   +10     
=============================================
+ Hits              33402    33412   +10     
  Misses             1087     1087           

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