fix(schema): ModelAlias variant alias target is never validated — empty or #-containing target yields a malformed ref #406

Problem

ModelAlias.resolveRef validates variants typed in the input and variants arriving via a model alias target (fixed for #398 in PR #403), but it never validates the value returned from the variant alias map (aliases.variants). A configured variant alias whose target is empty or contains a # produces a malformed or internally inconsistent ResolvedRef instead of the invalid-reference error every other path throws.

Evidence (verified 2026-10-24 on main c45147f)

File: packages/schema/src/model-alias.ts

• resolveVariant (lines 202-233) returns variantAliases[...] verbatim, with no check that the target is non-empty and #-free.
• The caller (lines 186-196) assigns that value straight to finalVariant and builds canonicalRef from it, so the bad value lands in both ResolvedRef.variant and ResolvedRef.ref.
• By contrast parseRefString (line 245) throws when a variant is empty or contains #, and the alias loop (lines 82-84) throws for the same shapes arriving in the input or via a model alias target.

Reproduction (Bun probe importing packages/schema/src/model-alias.ts, main c45147f):

Control: ModelAlias.resolve(undefined, "openai/gpt-4o#high#extra") throws Invalid model reference.

Why it matters

A typo or bad value in the variants alias map (user config) is silently turned into a canonical ref no other parser in the repo accepts, so it fails later — further from the cause — or is passed to a provider as a variant name that can never match. The empty-target shape is worse: resolution appears to succeed while quietly dropping the variant the user asked for, and ResolvedRef.variant === "" contradicts ResolvedRef.ref, which has no variant segment.

Acceptance criteria

• A variant alias target that is empty or contains # throws the same invalid-reference error as the no-alias path, naming the offending reference.
• ResolvedRef.variant is never "": it is either a non-empty variant or undefined, and it always agrees with ResolvedRef.ref.
• New tests in packages/core/test/model-alias.test.ts cover the #-containing and empty variant-alias-target shapes (bare and scoped/wildcard variant keys); all existing model-alias tests still pass.
• Dedup checked 2026-10-24: fix(schema): ModelAlias alias path accepts invalid variants the no-alias parser rejects — empty variant dropped, second # kept #398/PR fix(schema): validate variants on the alias path and settle chains at the depth limit #403 covered input variants and model-alias targets only; no open or closed issue tracks variant-alias (aliases.variants) targets.

Single-repo: ModelAlias lives only in this repo's schema/core packages.