Fix spurious circular-base-type diagnostics after invalid model is usage

[Copilot]

checkModelIs started a base-type pending resolution but did not always finish it on early error returns. This could leak pending state and cause unrelated later model is references to report a false circular-base-type diagnostic.

• Checker: close pending resolution on all error exits

  • Updated checkModelIs in packages/compiler/src/core/checker.ts to call:
    • pendingResolutions.finish(modelSymId, ResolutionKind.BaseType)
  • Added this before each early return undefined path:
    • is with a model expression
    • circular reference short-circuit in TypeReference path
    • non-model/non-array invalid is expression path

• Regression coverage: prevent duplicate/spurious diagnostics

  • Added focused tests in packages/compiler/test/checker/model.test.ts to verify invalid model is does not leave resolver state behind.
  • New cases assert only the expected is-model error is emitted when another model later references the invalid model.

model A is { x: int32 };
model B is A;
// Expect: only `is-model` (no spurious `circular-base-type`)

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commit: 089693e

[timotheeguerin]

@copilot add changelog

[Copilot]

@copilot add changelog

Added the changelog entry in .chronus/changes/copilot-fix-check-model-is-errors-2026-4-14-18-52-27.md in commit 8680bbb.

[azure-sdk-automation]

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[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler

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@typespec/compiler - fix ✏️

Fix spurious circular-base-type diagnostics after invalid 'model is' declarations.