[WAVE 2][RTR] Fix linear bounds rendering in plots

[mickaelbegon]

Summary

• render InterpolationType.LINEAR bounds with regular lines instead of post-steps
• preserve step rendering for constant and frame-based bounds
• add a focused regression test for interpolated minimum and maximum bounds

Root cause

Bounds were always sent through Axes.step(..., where="post"). This changes linear bounds into piecewise-constant values between shooting nodes, so they do not match integrated state trajectories such as collocation plots.

Validation

• MPLCONFIGDIR=/private/tmp/bioptim-mpl-973 python -m pytest tests/shard1/test_plot.py -q (6 passed)
• git diff --check

Closes #973

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This change is 

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.86%. Comparing base (89c83a3) to head (67e4534).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1065      +/-   ##
==========================================
+ Coverage   77.26%   77.86%   +0.60%     
==========================================
  Files         196      196              
  Lines       21506    21506              
==========================================
+ Hits        16616    16746     +130     
+ Misses       4890     4760     -130     

Flag	Coverage Δ
unittests	77.86% <100.00%> (+0.60%)	⬆️

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[mickaelbegon]

Codex analysis

This PR is part of Wave 2 and is rebased onto current master (e8fbc729). It limits the change to rendering linear plot bounds correctly and adds focused plot regression coverage.

Review isolation: it changes only bioptim/gui/plot.py and tests/shard1/test_plot.py; it has no source-file overlap with #1062 or #1067.

Validation completed by Codex:

• macOS, Python 3.14 / CasADi 3.7.2: tests/shard1/test_plot.py — 6 passed
• the unrelated macOS numerical test that failed on the first CI attempt was reproduced locally — 1 passed
• GitHub Actions after retrigger: 23/23 checks passed
• git diff --check passed

The final commit (67e45341) is intentionally empty and only retriggered CI after unrelated numerical-tolerance failures; it adds no review surface.

@EveCharbie @pariterre, all local and online checks are green. This PR is ready for review as an independent Wave 2 change.