(chore): generate 0.12.13 release notes (#2426)

Author: ilan-gold

docs/release-notes/0.12.13.md

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+(v0.12.13)=
+### 0.12.13 {small}`2026-05-07`
+
+#### Bug fixes
+
+- Ensure non-string indexes are preserved in {attr}`~anndata.AnnData.uns` using {func}`~anndata.experimental.read_lazy` (introduced by {pr}`2419`) {user}`ilan-gold` ({pr}`2423`)