MudForm: Base IsValid on required fields having a value, not being touched

[danielchalmers]

MudForm decided validity from requiredAllTouched, using Touched as a proxy for "required field satisfied".

Fix: base validity on whether each required field holds a value.

• IFormComponent.HasValue() — a default interface method (returns Touched, so existing external implementers are unaffected), overridden on MudFormComponent; empty strings and empty collections count as no value.
• MudForm uses noErrors && required.All(x => x.HasValue()) in both OnEvaluateForm and first-render init.

Behavior notes for upgraders:

• A pre-populated valid form is now valid on load; an empty required field is still invalid (no error shown until touched); an unselected required MudRadioGroup is now correctly invalid.
• A pre-populated value that would fail a custom Validation delegate reads as valid until it is validated (the delegate runs on interaction), and a debounced field with an uncommitted value is treated as having no value until it commits.

Related issues

• Inputs, Pickers: Only mark Touched and fire FieldChanged on user interaction #13328 (9.6.0) stopped marking Touched on parameter-driven value sets, so a pre-populated required field became HasValue=true but Touched=false — and the form read invalid on load (regressed from 9.5.0).
• Closes MudForm does not validate on load, if form is pre-populated #13421.
• Closes MudForm.IsValid Defaults to false with Required Fields #5196.
• Closes Required MudDatePicker not checked for bind-IsValid #12400.
• Closes MudForm: Fix IsValid initialization for components with Required=true #9793.
• Closes MudForm: Fix IsValid binding when required fields are empty on initial render #12664.

[danielchalmers]

Reproduced the actual consumer snippets in a live browser harness:

• Required int=0 → valid; int? null → invalid; int?=0 → valid.
• Select / MultiSelect / Autocomplete / Radio / DatePicker / TimePicker: preset → valid, empty → invalid.
• CheckBox/Switch unchecked → invalid (the BoolValue == true override holds — "must agree" still works); checked → valid.
• No-required-fields form → valid; ResetAsync clears a pre-filled required field → invalid.
• Exact #13421 register form (username + email/EmailAddressAttribute + radio + checkbox, all pre-filled): IsValid=true on load, Register button enabled — the regression is fixed.
• Failing-validator probe: a required field pre-filled "ab" with a custom validator requiring length ≥ 5 reads valid on load, but Submit → ValidateAsync() flips IsValid=false, disables the button, and shows the error. The valid-on-load state is transient and self-corrects on the standard submit path.

Two behaviours worth noting so they aren't misread as new bugs:

1. Validators run lazily. A pre-populated field whose value would fail a custom/DataAnnotation validator still reads valid on load — validators run on interaction or ValidateAsync(), exactly as before. A submit handler that calls ValidateAsync() (or MudForm's EditForm integration) catches it; Disabled="@(!IsValid)" alone was never a full guard.

2. Nested forms. A nested MudForm whose parent binds IsValid may report stale on first render until the child form's validity propagates; it corrects on the first field interaction. This is a pre-existing ChildForms aggregation quirk, orthogonal to this change, and is tracked separately.