[WAVE 1] Fix example content search paths

[mickaelbegon]

Summary

• preserve example subdirectories while flattening the launcher tree
• resolve content-search files from bioptim/examples instead of the process working directory
• add a regression test that validates every declared example path

Root cause

Content search combined os.getcwd() with only the top-level example category. Launching python -m bioptim.examples outside the examples directory therefore produced invalid paths, and nested toy_examples/... directories were discarded while flattening the tree.

Validation

• QT_QPA_PLATFORM=offscreen MPLCONFIGDIR=/private/tmp/bioptim-mpl-810 python -m pytest tests/shard1/test__run_examples.py::test_example_paths_are_preserved_when_flattening -q (1 passed)
• git diff --check

Closes #810

---

This change is 

[mickaelbegon]

Wave 1 preparation update: the Linux Qt import crash introduced by the new regression test is fixed in commit 1b3ac38 (focused test passes locally). Full CI still depends on the shared Matplotlib prerequisite #1081.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.86%. Comparing base (5ed90c1) to head (66ad492).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
bioptim/examples/__main__.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1068      +/-   ##
==========================================
+ Coverage   77.26%   77.86%   +0.60%     
==========================================
  Files         196      196              
  Lines       21506    21503       -3     
==========================================
+ Hits        16616    16743     +127     
+ Misses       4890     4760     -130     

Flag	Coverage Δ
unittests	77.86% <91.66%> (+0.60%)	⬆️

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