Fix inconsistent code-cell language specifications

[jstac]

Summary

• Fixed inconsistent code-cell language specifications in optimal savings lectures that caused jupytext UserWarnings
• Standardized language specs within each notebook for clean conversion

Changes

• os_egm_jax.md: Changed ipython → python3 (line 42 import cell had no IPython magics)
• os_stochastic.md: Changed ipython → python3 (line 65 import cell had no IPython magics)
• os_time_iter.md: Changed all cells to ipython for consistency (uses !pip install magic at line 28)

Testing

• Converted all modified files with jupytext - no UserWarnings
• Executed all generated .py files - all run successfully
• Verified no functional changes, only language specification corrections

🤖 Generated with Claude Code

[jstac]

All files from the last commit (1cb2924) were converted to Python using jupytext and tested successfully:

✅ os_egm.py - runs in ~0.03s
✅ os_egm_jax.py - runs in ~0.009s
✅ os_stochastic.py - runs successfully (takes ~1+ minute, converges in 229 and 237 iterations)
✅ os_time_iter.py - runs successfully (converges in 13-15 iterations)

All UserWarnings from jupytext have been eliminated with these language specification fixes.

[github-actions]

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📚 Changed Lecture Pages: os_egm, os_egm_jax, os_stochastic, os_time_iter