sonar(tools/corpus/seeder/SyntaxMutator.cs): 4 findings

[github-actions]

Filed by tools/sonar-triage/sonar-triage.ps1. This issue is the ticket: see docs/adr/0016-sonar-issue-triage.md.

Goal

Clear all 4 SonarCloud findings in tools/corpus/seeder/SyntaxMutator.cs.

Findings

Location	Rule	Message	Sonar
tools/corpus/seeder/SyntaxMutator.cs:158	csharpsquid:S125	Remove this commented out code.	open
tools/corpus/seeder/SyntaxMutator.cs:254	external_roslyn:IDE0300	Collection initialization can be simplified	open
tools/corpus/seeder/SyntaxMutator.cs:257	csharpsquid:S125	Remove this commented out code.	open
tools/corpus/seeder/SyntaxMutator.cs:401	csharpsquid:S3267	Loop should be simplified by calling Select(block => block.Statements))	open

Acceptance criteria

1. Every finding above is either fixed, or added to tools/sonar-triage/policy.jsonc with
   verdict: accept and a reason citing an ADR or a ticket. A finding that no longer
   reproduces on HEAD is called out in the PR body, not silently dropped.
2. .editorconfig pins dotnet_diagnostic.IDE0300.severity = error, so the build now fails on what was
   previously a silent suggestion and the finding cannot come back.
3. ./build.ps1 -Integration is green.
4. No #pragma warning disable, no coverage exclusion, no gate lowered.

How to fix

Point a session at this issue and say use equiv-sonar-fix.