Fix panic in getRestTypeAtPosition for contextually typed function with optional and rest params

[Copilot]

getRestTypeAtPosition panics with makeslice: len out of range when the source signature has fewer parameters than pos, producing a negative slice length.

const f: () => void = (a?, ...b) => {};  // panic

The context signature has 0 parameters, but assignContextualParameterTypes passes pos=1 (the non-rest param count of the function expression). The TS reference uses dynamic arrays with a for (let i = pos; i < parameterCount; i++) loop that naturally handles this—Go's pre-allocated make does not.

• Added a bounds check in getRestTypeAtPosition: when parameterCount - pos <= 0, return an empty tuple
• Added a minimal repro test case

[Copilot]

Pull request overview

Fixes a panic in the checker when computing a rest type at a parameter position beyond the contextual signature’s parameter count (e.g., context signature has 0 params but pos is 1), by avoiding negative slice lengths and returning an empty tuple rest type instead.

Changes:

• Add a guard in Checker.getRestTypeAtPosition to return an empty tuple when parameterCount - pos <= 0.
• Add a compiler test that reproduces the crash scenario (contextually typed function expression with optional + rest parameters).
• Add new reference baselines for the test (types/symbols/js/errors).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/relater.go	Prevents make([]T, negative) by handling out-of-range pos with an empty tuple rest type.
testdata/tests/cases/compiler/contextuallyTypedFunctionOptionalAndRest.ts	Adds a minimal repro case for the panic scenario.
testdata/baselines/reference/compiler/contextuallyTypedFunctionOptionalAndRest.types	Captures expected type output for the new test.
testdata/baselines/reference/compiler/contextuallyTypedFunctionOptionalAndRest.symbols	Captures expected symbol output for the new test.
testdata/baselines/reference/compiler/contextuallyTypedFunctionOptionalAndRest.js	Captures expected JS emit for the new test.
testdata/baselines/reference/compiler/contextuallyTypedFunctionOptionalAndRest.errors.txt	Captures expected diagnostics for the new test.