[ASM] Refactor hardcoded secret analyzer (#5883)

## Summary of changes

The Hardcoded analyzer has been refactored. These are the main changes:

- Replaced the manual sleep and polling mechanism with an asynchronous
delay using `Task.Delay`.
- Added `ConfigureAwait(false)` to avoid capturing the synchronization
context, which can prevent potential deadlocks and improve performance.
- Introduced a `CancellationTokenSource` for managing the lifecycle of
the polling operation.
- Removed the `_started` flag as the `CancellationToken` now controls
the start/stop logic.
- Ensured proper disposal of resources by calling
`_cancellationTokenSource.Cancel()` in the `Dispose` method.

## Reason for change

This change was requested.

## Implementation details

## Test coverage

## Other details
<!-- Fixes #{issue} -->

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Author: NachoEchevarria

tracer/src/Datadog.Trace/Iast/Analyzers/HardcodedSecretsAnalyzer.cs

@@ -23,28 +23,26 @@ internal class HardcodedSecretsAnalyzer : IDisposable
     private static readonly IDatadogLogger Log = DatadogLogging.GetLoggerFor<HardcodedSecretsAnalyzer>();
     private static HardcodedSecretsAnalyzer? _instance = null;
 
-    private readonly ManualResetEventSlim _waitEvent = new(false);
+    private readonly CancellationTokenSource _cancellationTokenSource = new();
     private readonly TimeSpan _regexTimeout;
-    private bool _started = false;
     private List<SecretRegex>? _secretRules = null;
 
     // Internal for testing
     internal HardcodedSecretsAnalyzer(TimeSpan regexTimeout)
     {
         Log.Debug("HardcodedSecretsAnalyzer -> Init");
         _regexTimeout = regexTimeout;
-        _started = true;
-        Task.Run(() => PoolingThread())
-                    .ContinueWith(t => Log.Error(t.Exception, "Error in Hardcoded secret analyzer"), TaskContinuationOptions.OnlyOnFaulted);
+        Task.Run(() => PollingThread(_cancellationTokenSource.Token))
+            .ContinueWith(t => Log.Error(t.Exception, "Error in Hardcoded secret analyzer"), TaskContinuationOptions.OnlyOnFaulted);
     }
 
-    private void PoolingThread()
+    private async Task PollingThread(CancellationToken cancellationToken)
     {
         try
         {
             Log.Debug("HardcodedSecretsAnalyzer polling thread -> Started");
             var userStrings = new UserStringInterop[UserStringsArraySize];
-            while (_started)
+            while (!cancellationToken.IsCancellationRequested)
             {
                 if (Tracer.Instance.Settings.IsIntegrationEnabled(IntegrationId.HardcodedSecret))
                 {
@@ -81,7 +79,7 @@ private void PoolingThread()
                                         IntegrationId.HardcodedSecret));
                                 }
                             }
-                            catch (Exception err)
+                            catch (Exception err) when (!(err is OperationCanceledException))
                             {
                                 Log.Warning(err, "Exception in HardcodedSecretsAnalyzer polling thread loop.");
                             }
@@ -94,12 +92,11 @@ private void PoolingThread()
                     }
                 }
 
-                _waitEvent.Wait(2_000);
+                await Task.Delay(2_000, cancellationToken).ConfigureAwait(false);
             }
         }
-        catch (Exception err)
+        catch (Exception err) when (!(err is OperationCanceledException))
         {
-            _started = false;
             Log.Warning(err, "Exception in HardcodedSecretsAnalyzer polling thread. Disabling feature.");
         }
 
@@ -215,12 +212,8 @@ private static List<SecretRegex> GenerateSecretRules(TimeSpan timeout)
 
     public void Dispose()
     {
-        try
-        {
-            _started = false;
-            _waitEvent.Set();
-        }
-        catch { }
+        _cancellationTokenSource.Cancel();
+        Log.Debug("HardcodedSecretsAnalyzer -> Disposed");
     }
 
     private readonly struct SecretRegex