Adds dynamic NuGet package resolution via #:package directives for C# file-based apps

[ashar-builds]

TLDR

Adds dynamic NuGet package resolution via #:package directives for C# file-based apps and fixes a double-insertion bug for zero-length completion spans.

Details

1. Support for #:package Directives:

• Added dynamic NuGet package restore capabilities in WorkspaceManager.SingleFile.cs and WorkspaceManager.cs.
• When a #:package directive is encountered in a C# script or file-based app, the sidecar generates a temporary .csproj, executes a background dotnet restore, and extracts the PortableExecutableReferences to load into the AdhocWorkspace.
• Implemented UpdateProjectlessClosureAsync to support real-time graph and package updates. As the user edits the document, new packages and included files are seamlessly resolved and injected into the active solution.

2. Autocomplete Double-Insertion Fix:

• Fixed ComputeCompletionEditSpan overlap logic: Replaced OverlapsWith with IntersectsWith in WorkspaceManager.Helpers.cs.
• Previously, zero-length completion spans (e.g., triggering completion immediately after typing a .) were failing the overlap check, causing the primary text edit to be incorrectly appended to the AdditionalEdits array alongside the main edit.
• Cleaned up temporary workarounds in src/semantic.rs and WorkspaceManager.Helpers.cs to ensure standard LSP completion mechanisms are maintained without stripping insert_text or filter_text.

How Do The Automated Tests Prove It Works?

• New Test Added: Added ResolveCompletion_with_empty_span_skips_primary_edit_in_additional_edits to WorkspaceManagerFeatureCoverageTests.cs. This test simulates the exact bug scenario by triggering completion at a 0-length span immediately following a dot (result.). It asserts that the primary completion edit (e.g., "ToString") is successfully filtered out and does not appear in the AdditionalEdits array.
• Full Suite Verification: The C# Sidecar tests pass perfectly (466/466) and the Rust host tests pass (309/309 feature tests), proving this fix cleanly resolves the issue without regressing existing completion, navigation, or scripting features.