Use range indexer in SubstringAfter

[Copilot]

• Replace str.Substring(index + sub.Length, str.Length - index - sub.Length) with str[(index + sub.Length)..] in SubstringAfter
• Fix formatting to match SubstringUntil style (single-line ternary, no blank line)

Original prompt

Problem

In YoutubeExplode/Utils/Extensions/StringExtensions.cs, the SubstringAfter method uses the old string.Substring(startIndex, length) overload, which requires manually computing the length:

public string SubstringAfter(
    string sub,
    StringComparison comparison = StringComparison.Ordinal
)
{
    var index = str.IndexOf(sub, comparison);

    return index < 0
        ? string.Empty
        : str.Substring(index + sub.Length, str.Length - index - sub.Length);
}

This can be simplified to use the range indexer str[(index + sub.Length)..], which is cleaner and consistent with how SubstringUntil is already written in the same file (str[..index]).

Fix

Replace the Substring(startIndex, length) call with a range slice:

public string SubstringAfter(
    string sub,
    StringComparison comparison = StringComparison.Ordinal
)
{
    var index = str.IndexOf(sub, comparison);

    return index < 0
        ? string.Empty
        : str[(index + sub.Length)..];
}

No functional behavior changes.

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[Tyrrrz]

@copilot formatting issues

[Copilot]

Pull request overview

This PR modernizes the SubstringAfter method to use C# range indexer syntax, making it consistent with the sibling SubstringUntil method and the broader codebase patterns.

Changes:

• Replaced verbose Substring(startIndex, length) call with cleaner range indexer syntax str[(index + sub.Length)..] in the SubstringAfter extension method

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[Copilot]

@copilot formatting issues

Fixed in 6e09820 — collapsed SubstringAfter to a single-line ternary with no blank line, matching the style of SubstringUntil.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (31cefef) to head (6e09820).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@      Coverage Diff      @@
##   master   #941   +/-   ##
=============================
=============================

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