Fix mapped type hover nil dereference

[Copilot]

Description

Hovering a function whose parameter contains { [K in keyof T] } panics while printing the signature because the mapped property type is absent.

• Printer: Emit : Type only when the mapped-type property type exists.
• Regression coverage: Add a fourslash hover baseline for the failing declaration.

declare function uhoh<T>(x: { [K in keyof T] }): void;

• Fixes Nil dereference when printing back function that has mapped type with no mapped property type #64111

[Copilot]

Pull request overview

Prevents mapped-type hover crashes when the property type is absent.

Changes:

• Guard mapped-type property type printing against nil.
• Add fourslash hover regression coverage and baseline.

Show a summary per file

File	Description
tsc/internal/printer/printer.go	Conditionally emits mapped property types.
tsc/internal/fourslash/tests/hoverMappedTypePropertyJSDoc_test.go	Adds hover regression test.
tsc/testdata/baselines/reference/fourslash/quickInfo/hoverMappedTypeWithoutPropertyType.baseline	Records expected hover output.

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• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Balanced