fix(combo): drop malformed custom models from the auto candidate pool

[diegosouzapw]

Re-cut of #15803 onto the active cycle (v3.8.51 is shipped).

What

customModels is an operator-writable key_value JSON blob. expandAutoComboCandidatePool now drops rows that are not objects with a non-empty string id (and a string[] supportedEndpoints when one is present) before the chat endpoint policy runs. Before this, a malformed row (null, a bare string, { name } without an id) could abort the expansion or make the provider fall back to its static catalogue instead of the operator's visible custom chat model.

Carried / dropped vs #15803

• Carried: the sanitize half: isValidAutoCustomModel / sanitizeAutoCustomModels in open-sse/services/combo/autoStrategy.ts, plus its regression test in tests/unit/combo-auto-candidate-expansion.test.ts.
• Dropped: the typing half. It is already on release/v3.8.52 via fix(combo): type the custom-model row in the auto pool check #15461.
• Small difference: the helpers sit above the expandAutoComboCandidatePool JSDoc, not between the JSDoc and the function, so the docblock stays attached to the function.

Test evidence

• node --import tsx/esm --test tests/unit/combo-auto-candidate-expansion.test.ts
  • with the v3.8.52 tip autoStrategy.ts: 9/10, 1 failed: ignores malformed custom rows while keeping chat models → AssertionError: only the well-formed visible custom chat model should be expanded
  • with the change (after rebasing onto f90e64a): 10/10 pass
• prettier --check, eslint --suppressions-location config/quality/eslint-suppressions.json on the changed files, npm run typecheck:core, npm run check:open-sse-typecheck (0 errors) and npm run typecheck:noimplicit:core (0 errors in autoStrategy.ts) → clean.

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