fix(inputtext,datepicker): remove unreachable undefined from emit types

[YevheniiKotyrlo]

Defect Fixes

Fixes #8441

InputText and DatePicker declare | undefined in their update:modelValue and value-change emit signatures, but neither component can emit undefined at runtime. With strictTemplates: true, the emitted | undefined is not assignable to the consumer's ref type, producing 39 false-positive type errors that block adoption of strictTemplates.

Related: #6041

Reproducer: StackBlitz — run bun run type-check in terminal (TS2322 false positive), then bun run patch && bun run type-check (0 errors)

Problem

<InputText v-model="name" />
<!-- name: Ref<string> -->
<!--
  error TS2322: Type 'string | undefined' is not assignable to type 'string'.
  38 occurrences across a typical project — all false positives
-->

After this fix, v-model correctly matches the consumer's type — no error.

Component	Emit signature before	Emit signature after	Consumer ref	Errors fixed
InputText	string | undefined	string	Ref<string>	38
DatePicker	Date | ... | undefined | null	Date | ... | null	Ref<Date | null>	1

Root cause

InputText: Single emission path — onInput(event) -> writeValue(event.target.value). HTMLInputElement.value is always string per HTML spec. Full inheritance chain audited — no resetValue(), clear(), or watcher re-emission exists. No path can emit undefined.

DatePicker: All 9 emission paths traced through updateModel() -> writeValue() -> $emit(). Clear button emits null. Invalid text input throws (no emission). Range mode maps undefined -> null before calling formatDateTime. 7 edge cases pressure-tested (empty input, invalid text, range incomplete, multi deselect). undefined is never passed to writeValue().

Changes

• inputtext/InputText.d.ts: string | undefined -> string (2 emit signatures)
• datepicker/DatePicker.d.ts: remove | undefined, preserve | null (2 emit signatures)

Scope / Impact

• No breaking changes — removes an impossible type from emit signatures
• No runtime changes — types only (.d.ts files)
• Eliminates 39 false-positive errors blocking strictTemplates adoption

Verification

Gate	Result
InputText source audit	Single emission path — only emits string
DatePicker source audit	All 9 paths traced — none emit undefined
DatePicker edge cases	7 edge cases pressure-tested
vue-tsc --noEmit with strictTemplates	39 errors eliminated, 0 introduced
All CI gates (format, lint, test, build)	PASS