Migrate ResolveCodeAnalysisRuleSet to multithreaded execution

[jankratochvilcz]

Migrates ResolveCodeAnalysisRuleSet to support MSBuild's multithreaded execution model.

Changes

• [MSBuildMultiThreadableTask] applied; class implements IMultiThreadableTask with TaskEnvironment = TaskEnvironment.Fallback default.
• Ruleset and include-directory path resolution routed through TaskEnvironment.GetAbsolutePath.

Compatibility audit

Ran the 6-deadly-sins playbook (see .github/skills/multithreaded-task-migration/SKILL.md):

• Sin 1 ([Output] contamination): ResolvedCodeAnalysisRuleSet is assigned the original CodeAnalysisRuleSet value or the unaltered Path.Combine(directory, CodeAnalysisRuleSet) joined string — no AbsolutePath is ever leaked into the output. Absolutization is used solely for the File.Exists probes.
• Sin 2 (error message inflation): The Compiler.UnableToFindRuleSet warning continues to be formatted with the original CodeAnalysisRuleSet user input.
• Sin 3 (?? swallowing exceptions): N/A — no null-coalescing was introduced.
• Sin 4 (try-catch scope): N/A — no try/catch in this task.
• Sin 5 (canonicalization): N/A — paths are not used as dictionary/set keys, and the previous code did not call Path.GetFullPath.
• Sin 6 (empty/null inputs): Empty/null CodeAnalysisRuleSet is short-circuited at the top before any GetAbsolutePath call, preserving the original return null behavior. Empty MSBuildProjectDirectory is still gated by !string.IsNullOrEmpty so we never call GetAbsolutePath with a degenerate combined path.

Validation

• Microsoft.Build.Tasks.csproj builds clean (0 warnings, 0 errors).
• ResolveAnalyzerRuleSet_Tests passes on both net10.0 and net472 (12 tests × 2 TFMs = 24 passing).

Part of #11834. Closes #13569.