btquotients: make local splitting optimal.

[Eloitor]

This PR ensures that the local splitting is optimal with respect to the maximal order.

When computing a local splitting without magma, the splitting was not checked to be optimal. This produced a ValueError bug, when the image of some element of the basis of the maximal order had negative valuation.

Fixes #40209

For example:

sage: X = BruhatTitsQuotient(7, 5*17*13)
sage: X.genus_no_formula()
385
sage: X.genus()
385

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

[github-actions]

Documentation preview for this PR (built with commit 803c317; changes) is ready! 🎉
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[Eloitor]

ready for review

[Eloitor]

If I'm not wrong this could be simplified by #23486

[fchapoton]

linter is not happy

[Eloitor]

I don't know how to fix it: src/sage/modular/btquotients/btquotient.py:2206:1: RST218 Literal block expected; none found.

[fchapoton]

I have fixed it for you