Fix Config Source Gen to avoid compilation error missing System.Collections.Generic namespace

[tarekgh]

Fixes #118630

The configuration source generator can produce code that references collection types such as HashSet without including the System.Collections.Generic namespace. This results in a compilation error. More users have started encountering this issue when using project templates like webapiaot and disabling ImplicitUsings.

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[tarekgh]

CC @ezhevita

[tarekgh]

CC @jeffhandley @ericstj

[Copilot]

Pull Request Overview

This PR moves the System.Collections.Generic namespace import to the initial namespace set in the binding helper code generator, removing conditional logic that previously added it only when processing object types.

• Adds System.Collections.Generic to the default namespace set initialized in BindingHelperInfo.Builder
• Removes the conditional namespace addition that occurred when registering object types
• Updates all baseline test files to reflect the new namespace being consistently included
• Adds a new test case for binding to collection-only scenarios

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated no comments.

File	Description
src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Specs/BindingHelperInfo.cs	Moves System.Collections.Generic to the initial namespace set and removes conditional addition logic
src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/GeneratorTests.cs	Adds new test for binding to collection types only
src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/Baselines/netcoreapp/ConfigurationBinder/Version1/*.generated.txt	Updates baseline files to include the new namespace
src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/Baselines/net462/ConfigurationBinder/Version1/*.generated.txt	Updates baseline files to include the new namespace

[jeffhandley]

/ba-g Timeouts in Android and known test failures

[jeffhandley]

@tarekgh Since customers are reporting this against .NET 10 RC bits and the scenario is becoming more common, I would support submitting for servicing consideration, illustrating the webapiaot scenario, the code that triggers the error, and the workaround that you prescribed in the issue. The risk for the fix looks low.

[tarekgh]

/ba-g the failures are known timeout issues.