Pickers: Fix ReadOnly not respected in PickerVariant.Static

[Copilot]

MudDatePicker, MudDateRangePicker, and MudTimePicker with PickerVariant.Static and ReadOnly="true" still allowed date/time selection. Although SubmitAsync guarded against persisting values, the intermediate selection state (_selectedDate, _firstDate, _secondDate, TimeIntermediate) was set before the guard, causing visual changes and actual value mutations in the static variant.

Changes

• MudDatePicker — Early return on GetReadOnlyState() in OnDayClickedAsync, OnMonthSelectedAsync, and OnYearClickedAsync
• MudDateRangePicker — Early return on GetReadOnlyState() in OnDayClickedAsync
• MudTimePicker — Early return on GetReadOnlyState() in UpdateTimeAsync (shared entry point for both pointer-move and stick-click interactions)

<MudDatePicker PickerVariant="PickerVariant.Static" ReadOnly="true" @bind-Date="_date" />
<!-- Clicking any day no longer changes _date or the visual selection state -->

Three unit tests added covering the static + ReadOnly scenario for each affected picker.

Original prompt

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This section details on the original issue you should resolve

<issue_title>MudDatePicker not respecting ReadOnly in PickerVariant.Static</issue_title>
<issue_description>### Bug type

Component

Component name

MudDatePicker

What happened?

When using a DatePicker in static mode, it doesn't respect ReadOnly, when setting ReadOnly to true, it still lets you interact with the control and select a date.

Expected behavior

No date selection possible when readonly.

Reproduction link

https://try.mudblazor.com/snippet/wkcxaUbCPPrHTiyy

Reproduction steps

1. Add DatePicker
2. Set to PickerVariant.Static
3. Set to ReadOnly
4. Interaction is still occuring.

Relevant log output

No response

Version (bug)

6.4.1

Version (working)

No response

What browsers are you seeing the problem on?

Firefox, Chrome, Microsoft Edge, Safari, Other

On what operating system are you experiencing the issue?

Windows, Mac OSX, Linux, iOS, Android, Other

Pull Request

• I would like to do a Pull Request

Code of Conduct

• I agree to follow this project's Code of Conduct</issue_description>

<agent_instructions>Fix this on each picker in a minimal and targeted way</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@danielchalmers @binbsr Niche issue so it's unlikely to be fixed unless someone submits a PR

• Fixes MudDatePicker not respecting ReadOnly in PickerVariant.Static #7038

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