Add STJ path in AutoCompleteResourceV2Feed

[Nigusu-Allehu]

Bug

Fixes: NuGet/Home#14895

Description

Adds a feature-flagged System.Text.Json deserialization path for V2 feed autocomplete results in AutoCompleteResourceV2Feed. The existing NSJ path is unchanged.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

[Copilot]

Pull request overview

Adds a System.Text.Json deserialization path to AutoCompleteResourceV2Feed.GetResults, gated behind the existing NuGet.UseSystemTextJsonDeserialization feature switch / NUGET_USE_SYSTEM_TEXT_JSON_DESERIALIZATION env var, mirroring the pattern already used by AutoCompleteResourceV3, ServiceIndexResourceV3Provider, and RepositorySignatureResourceProvider. The V2 autocomplete endpoints return a top‑level JSON string[], so string[] is added to the shared JsonContext source-generation context and the existing tests are parameterized to cover both code paths.

Changes:

• Adds an internal constructor on AutoCompleteResourceV2Feed accepting an IEnvironmentVariableReader and branches GetResults between STJ and Newtonsoft.Json based on the feature flag/env var.
• Registers [JsonSerializable(typeof(string[]))] on JsonContext.
• Converts the three AutoCompleteResourceV2FeedTests cases to [Theory] with "true"/"false" data, constructing the resource directly with a mocked env reader.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/NuGet.Core/NuGet.Protocol/LegacyFeed/AutoCompleteResourceV2Feed.cs	Adds STJ branch in GetResults and an internal constructor taking IEnvironmentVariableReader.
src/NuGet.Core/NuGet.Protocol/Utility/JsonContext.cs	Adds string[] to the STJ source-generation context for V2 autocomplete responses.
test/NuGet.Core.Tests/NuGet.Protocol.Tests/AutoCompleteResourceV2FeedTests.cs	Parameterizes existing tests over the STJ/NSJ feature switch using a mocked env reader.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[nkolev92]

one question that may or may not lead to a code change :D

Approving in case my hunch is wrong.