perf: Cache SemanticModel and improve null handling in generators

[thomhurst]

Summary

• Caches SemanticModel in AsyncModuleCodeFixProvider instead of calling GetSemanticModelAsync multiple times
• Improves null handling consistency in OptionsClassGenerator GeneratorUtils

Issues Fixed

• Closes Code smell: Duplicate GetSemanticModelAsync calls in AsyncModuleCodeFixProvider #1572 - Duplicate GetSemanticModelAsync calls in AsyncModuleCodeFixProvider
• Closes Code smell: OptionsClassGenerator has inconsistent null handling for options #1639 - OptionsClassGenerator has inconsistent null handling for options

Changes

• AsyncModuleCodeFixProvider.cs: Caches SemanticModel, makes helper methods static
• GeneratorUtils.cs: Uses nullable parameters with consistent empty string return

Test plan

• Build passes for both Analyzers and OptionsGenerator solutions

🤖 Generated with Claude Code

[thomhurst]

Summary

This PR optimizes the AsyncModuleCodeFixProvider by caching the semantic model to avoid redundant async calls, and improves null safety in GeneratorUtils methods.

Critical Issues

None found ✅

Suggestions

AsyncModuleCodeFixProvider.cs - Consider CancellationToken

The cached semantic model at line 64 doesn't pass the cancellationToken parameter:

var semanticModel = await context.Document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);

This is good - the cancellation token is properly threaded through.

GeneratorUtils.cs - Behavioral Change in EscapeIdentifier

The change at line 163 modifies behavior:

• Before: EscapeIdentifier(null) or EscapeIdentifier("") → returns the input (null or empty string)
• After: Returns string.Empty in both cases

This is technically a breaking change for callers expecting null preservation, though current usage appears safe. Consider documenting this behavior change in the method's XML documentation.

Suggested addition:

/// <summary>
/// Escapes a C# identifier if it's a reserved keyword by prefixing with @.
/// Returns an empty string if the input is null or empty.
/// </summary>

Verdict

✅ APPROVE - No critical issues

The performance optimization in AsyncModuleCodeFixProvider is well-implemented and the null safety improvements are beneficial. The behavioral change in EscapeIdentifier appears safe based on current usage patterns.