#43496 added a per-provider hide-all sentinel ('provider::') so emptying a provider in the Edit Models dialog stopped re-expanding its defaults. That fixed the single-provider case, but the dialog's toggle handler seeds its working set from effectiveVisibleKeys(), which strips ALL sentinels before returning. So persisting after any toggle silently dropped every OTHER provider's hide-all sentinel; those providers then looked 'never customized' and re-enabled all their models on the next render.
Split resolution into two functions:
- resolveVisibleKeys(): stored keys + curated default expansion, with hide-all sentinels PRESERVED — the canonical working set the toggle handler mutates and persists.
- effectiveVisibleKeys(): resolveVisibleKeys() then strips sentinels, for display only (unchanged contract).
Move the toggle set-computation into a pure, unit-tested toggleModelVisibility() that seeds from resolveVisibleKeys(), so sibling sentinels survive the persist. Add regression tests that drive the real toggle handler across multiple providers.
Follow-up to #43496; completes the fix for #43485 (cross-provider case).
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