feat: Add PH2161 analyzer to avoid [Retry] attribute and RetryBaseAttribute derivatives

[Copilot]

This PR implements a new analyzer PH2161 that detects and prevents the use of MSTest's [Retry] attribute and any custom attributes that derive from RetryBaseAttribute.

Problem

MSTest provides a [Retry] attribute to automatically retry flaky tests, but this approach masks underlying issues rather than fixing them. As stated in the issue, "it would be preferable to avoid the flaky test" rather than attempting to overcome it with retries.

Solution

The new analyzer detects two scenarios:

1. Direct usage of [Retry] attribute:

[TestMethod]
[Retry(3)]  // ❌ PH2161: Retry attribute not allowed
public void TestMethod() { }

2. Custom attributes deriving from RetryBaseAttribute:

public class CustomRetryAttribute : RetryBaseAttribute
{
    public CustomRetryAttribute(int count) : base(count) { }
}

[TestMethod]
[CustomRetry(5)]  // ❌ PH2161: Tests may not use attributes that derive from RetryBaseAttribute
public void TestMethod() { }

Implementation Details

• Diagnostic ID: PH2161
• Severity: Error
• Enabled by default: Yes
• Category: MsTest

The implementation extends the existing AvoidAttributeAnalyzer with:

• Direct attribute name checking for "Retry"
• Semantic model analysis to detect inheritance from RetryBaseAttribute
• Logic to avoid double-reporting when the same attribute matches both checks

Testing

Added comprehensive test coverage including:

• Direct [Retry] attribute usage detection
• Custom attributes deriving from RetryBaseAttribute detection
• Verification that non-retry attributes are ignored
• All existing tests continue to pass (2013 total)

Documentation

Created complete documentation at Documentation/Diagnostics/PH2161.md following the established pattern, including examples and guidance on how to resolve violations.

Fixes #976.

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[Brian Collamore (bcollamore)]

Let's switch to 2161. (Another PR has claimed 2159 and 2160). We'll need to update our PH2159.md file name and contents as well.

[Copilot]

Updated to use PH2161 as requested. Changed the diagnostic ID in DiagnosticId.cs and AvoidAttributeAnalyzer.cs, renamed the documentation file from PH2159.md to PH2161.md, and updated all references. All 2013 tests pass. (commit 5d8c238)

[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

[Brian Collamore (bcollamore)]

Copilot see comments above

[Copilot]

Copilot see comments above

Addressed the diagnostic ID change from PH2159 to PH2161 as requested in the comment above. All changes have been made and validated. (commit 5d8c238)