Add missing #pragma warning disable to generated code headers

[Skymly]

Summary

• Add #pragma warning disable CS1591,CS8019,CS0162,CS0612,CS0618 to the auto-generated header trivia in GeneratedCodeHelper.CreateAutoGeneratedHeader. This is emitted before the using directives and applies to all generated code.
• The XML doc on GeneratedCodeHelper claimed to add #pragma warning disable but the implementation had none. Generated code can trigger warnings (CS1591 missing XML doc, CS8019 unused using, CS0162 unreachable code, CS0612/CS0618 obsolete) that become errors in consumer projects with TreatWarningsAsErrors=true.
• 55 Verify snapshots updated (Verify strips leading trivia in its default scrubber, so the #pragma line is not visible in snapshots but is present in the actual generated source — verified via a temporary assertion test).

Suppressed warning codes

Code	Reason
CS1591	Missing XML doc on generated internal/private members
CS8019	Unnecessary using directive (TFM-conditional usings)
CS0162	Unreachable code (conditional #if paths)
CS0612	Obsolete member (non-formatted)
CS0618	Obsolete member (formatted)

Related Issue

Closes #209

Solution module

• SourceGenerators (DesignPatterns.SourceGenerators/)

Type of change

• Bug fix

Test plan

• dotnet build DesignPatterns.slnx -c Release — 0 errors, 0 warnings
• dotnet test DesignPatterns.slnx -c Release --no-build — all tests pass
• Temporary assertion test confirmed #pragma warning disable present in generated source text (test removed before commit)
• 55 Verify snapshots updated

Breaking changes

• None

[Skymly]

Review: PR #210 — Add #pragma warning disable to generated code headers

Clean implementation. Adds #pragma warning disable CS1591,CS8019,CS0162,CS0612,CS0618 to the auto-generated header, suppressing warnings that generated code may trigger but consumers shouldn't see.

What's good

• PragmaWarningDisableCodes constant centralizes the warning codes — easy to maintain.
• Warning code selection is reasonable:
  • CS1591 — missing XML doc comment (generated code doesn't have XML docs)
  • CS8019 — unnecessary using directive (generated code may include usings that aren't always needed)
  • CS0162 — unreachable code (generated code may have conditional branches)
  • CS0612 / CS0618 — obsolete member usage (generated code may reference APIs marked obsolete)
• ParseLeadingTrivia correctly parses the pragma directive with newline.
• All Verify snapshots updated to include the new pragma line.

Observation

• The pragma is added to the header trivia (before #nullable enable), which means it's in scope for the entire generated file. This is correct — the pragma should cover all generated code.
• Note: this PR and PR #208 both modify GeneratedCodeHelper.cs and the same verified.txt files. They will need to be merged sequentially with a rebase in between.

No blocking issues. Approve.