feat(Select): redesign IsClearable function

[trycatchfinnally]

Issue

fixes #5629

对于引用类型，或可空值类型，清除选中项应将选中值设置为null，而不应是取默认第一项的值，目前，主流ui框架均支持直接清除，建议增加此功能
此部分代码涉及到select组件 GetSelectedRow和OnClearValue方法

Summary by Sourcery

This pull request includes several changes to the Select component and its related files in the BootstrapBlazor project. The most important changes involve enhancements to the Select component's clearable functionality, updates to the Selects.razor sample, and improvements to the unit tests.

Enhancements to Select component:

• Modified the GetClearable method to ensure IsClearable only works if the value is nullable. (src/BootstrapBlazor/Components/Select/Select.razor.cs)
• Added a private method IsNullable to check if the value type is nullable. (src/BootstrapBlazor/Components/Select/Select.razor.cs)

Updates to Selects.razor sample:

• Reorganized and expanded the IsClearable demo block with new examples and descriptions. (src/BootstrapBlazor.Server/Components/Samples/Selects.razor)
• Introduced a new MockModel class to support nullable properties in the IsClearable demo. (src/BootstrapBlazor.Server/Components/Samples/Selects.razor.cs)
• Added new localized strings for the IsClearable demo description in both English and Chinese. (src/BootstrapBlazor.Server/Locales/en-US.json, src/BootstrapBlazor.Server/Locales/zh-CN.json) [1] [2]

Improvements to unit tests:

• Enhanced the IsClearable_Ok test to check for nullable and non-nullable types. (test/UnitTest/Components/SelectTest.cs) (Ff1c59c4L154R175)
• Added a new IsNullable_Ok test to verify the IsNullable method for different data types. (test/UnitTest/Components/SelectTest.cs)
• Updated existing tests to reflect changes in clearable functionality and nullable checks. (test/UnitTest/Components/SelectTest.cs) [1] [2]

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request modifies the Select component to correctly handle nullable value types when the IsClearable property is set. The GetSelectedRow and OnClearValue methods were updated to ensure that clearing the selection sets the value to null instead of the default first item when the bound type is nullable. A new ValueCanBeNull method was added to determine if the generic type TValue is nullable.

Sequence diagram for OnClearValue when ValueCanBeNull is true

sequenceDiagram
    participant SelectComponent
    SelectComponent->SelectComponent: OnClearValue()
    SelectComponent->SelectComponent: ValueCanBeNull()
    activate SelectComponent
    SelectComponent->SelectComponent: Set SelectedItem to null
    SelectComponent->SelectComponent: Set CurrentValueAsString to string.Empty
    SelectComponent->SelectComponent: Set Value to default(TValue)
    SelectComponent->SelectComponent: Invoke ValueChanged(default(TValue))
    deactivate SelectComponent

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File-Level Changes

Change	Details	Files
Modified the GetSelectedRow method to consider nullable value types when selecting the default item.	Added a check for whether the value type can be null. Modified the default item selection logic to prevent selection when the value can be null.	src/BootstrapBlazor/Components/Select/Select.razor.cs
Implemented logic in OnClearValue to handle nullable value types, setting the selected value to null when the component is clearable and the value type is nullable.	Added a check for whether the value type can be null. If the value type can be null, set SelectedItem to null and update the CurrentValueAsString and Value properties to their default values. Removed the logic to select the first available item when clearing the selection for nullable types.	src/BootstrapBlazor/Components/Select/Select.razor.cs
Added a ValueCanBeNull method to determine if the generic type TValue is nullable.	Added a new private method ValueCanBeNull. The method checks if TValue is a value type and if it's a nullable type using Nullable.GetUnderlyingType.	src/BootstrapBlazor/Components/Select/Select.razor.cs

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[bb-auto]

Thanks for your PR, @trycatchfinnally. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[sourcery-ai]

Hey @trycatchfinnally - I've reviewed your changes - here's some feedback:

Overall Comments:

• Consider extracting the logic within OnClearValue into smaller, well-named methods to improve readability.
• The ValueCanBeNull method could be a useful extension method on Type.

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟢 Testing: all looks good
• 🟡 Complexity: 1 issue found
• 🟢 Documentation: all looks good

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[trycatchfinnally]

@dotnet-policy-service agree

[trycatchfinnally]

@microsoft-github-policy-service agree

[kimdiego2098]

大佬们，提个建议，既然增加了这一项，那也可以再加一个配置属性，AutoAddNullItem , 启用时把Null项加到列表里

[ArgoZhang]

@sourcery-ai review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (79fce04) to head (37d3326).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #5626   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          653       653           
  Lines        29344     29342    -2     
  Branches      4176      4180    +4     
=========================================
- Hits         29344     29342    -2     

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