fix(providers/fab): pass role name string to delete_role instead of Role object

[YoannAbriel]

FABAuthManagerRoles.delete_role() passes the Role ORM object to security_manager.delete_role() which expects a string. SQLAlchemy then tries to use the Role object as a SQL literal in the WHERE clause, raising ArgumentError: Object TestRole is not legal as a SQL literal value.

Fix: pass existing.name (the role name string) instead of the Role object.

Closes: #63336

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 4, claude-opus-4-6)

Generated-by: Claude Code (Opus 4, claude-opus-4-6) following the guidelines

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