refactor: Convert CodeCovSettings from class to record (#1719)

Convert CodeCovSettings to a record for consistency with other settings
classes (CodacySettings, GitHubSettings, NuGetSettings, PublishSettings).
Also adds [SecretValue] attribute to Token property to match the pattern
used by other settings that contain sensitive values.

Fixes #1546

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: thomhurst

src/ModularPipelines.Build/Settings/CodeCovSettings.cs

@@ -1,6 +1,9 @@
+using ModularPipelines.Attributes;
+
 namespace ModularPipelines.Build.Settings;
 
-public class CodeCovSettings
+public record CodeCovSettings
 {
+    [SecretValue]
     public string? Token { get; init; }
-}
+}