Deduplicate the delimiter-separated casing validators

[matt-edmondson]

Follow-up to #158, which merged while this last piece was still in flight.

#158 failed the quality gate on new_duplicated_lines_density (9.0% against a 3% threshold). The cause was pre-existing, not introduced: the kebab, snake and macro validators were 44–60% duplicated against each other as whole files, so touching any line inside those blocks counts as new duplicated code. Coverage on that PR reached 100%, and all three quality ratings were A — duplication was the only remaining condition.

All three validators implement the same five rules and differ in exactly three values: the delimiter, the delimiter belonging to a different convention that must not appear, and the required letter case. That is now DelimitedCaseValidation.Validate, leaving each validator a single delegating expression:

protected override ValidationResult ValidateValue(string value) =>
    DelimitedCaseValidation.Validate(value, -, _, char.IsLower, FailureMessage);

Net -102/+68 lines, and the three files drop from ~50% duplicated to none.

Behaviour is unchanged. One incidental improvement: the consecutive-delimiter check is an index scan rather than Contains("--"), which drops a string allocation per call and avoids constructing the doubled delimiter at run time.

This was only safe to do because #158 added tests for the two rejection branches — a foreign separator, and correct separators with the wrong letter case — that had never been exercised for any of the three. Every rule in the shared helper is now covered.

Verification

• dotnet build — 0 errors, 0 warnings, all target frameworks
• dotnet test — 1088/1088 pass

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud