feat: HTML test report generation

[thomhurst]

Closes #4887. Adds HtmlReporter with --report-html option.

[claude]

Code Review

Overall, this is a clean, well-structured implementation that follows the existing JUnitReporter pattern closely. The approach is solid — opt-in via CLI flags, self-contained HTML with no external dependencies, and correct HTML encoding of user-controlled data. No bugs were found.

A few suggestions for improvement:

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1. Use BCL's WebUtility.HtmlEncode instead of a custom implementation

HtmlReporter.cs lines 437–450:

private static string HtmlEncode(string? value)
{
    if (string.IsNullOrEmpty(value)) return string.Empty;
    return value
        .Replace("&", "&")
        .Replace("<", "&lt;")
        .Replace(">", "&gt;")
        .Replace("\"", "&quot;")
        .Replace("'", "&#39;");
}

Why this matters: The chained .Replace() calls allocate 4 intermediate strings per invocation, and the method is called multiple times per test row. More importantly, rolling a custom HTML encoder risks edge-case misses over time. System.Net.WebUtility.HtmlEncode() is already in the BCL (no new dependencies), handles a broader set of characters correctly, and produces a single allocation. Suggested replacement:

private static string HtmlEncode(string? value)
    => string.IsNullOrEmpty(value) ? string.Empty : System.Net.WebUtility.HtmlEncode(value);

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2. ConsumeAsync should return ValueTask per CLAUDE.md performance guidelines

HtmlReporter.cs lines 165–173:

public Task ConsumeAsync(IDataProducer dataProducer, IData value, CancellationToken cancellationToken)
{
    ...
    return Task.CompletedTask;
}

CLAUDE.md Rule 4 (Performance First) states: "Use ValueTask for potentially-sync operations." This method is always synchronous and returns Task.CompletedTask, which causes a heap allocation on every test result update (a hot path during test execution). If the IDataConsumer interface in Microsoft.Testing.Platform supports ValueTask, this should be changed. (Note: JUnitReporter.cs has the same pattern — fixing it here and there in tandem would be ideal.)

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3. Modern C# index syntax (^1)

HtmlReporter.cs line ~194:

lastUpdates[kvp.Key] = kvp.Value[kvp.Value.Count - 1];

CLAUDE.md requires modern C# features. C# 8+ supports ^1 for last-element access:

lastUpdates[kvp.Key] = kvp.Value[^1];

(Same pre-existing pattern exists in JUnitReporter.cs.)

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None of these are blocking — the implementation is correct and the overall design is good.