## Summary of changes

This resolves a `System.InvalidCastException` that was happening in the
GRPC client instrumentation (I think just GrpcLegacy) when we attempted
to remove headers.


## Reason for change

Errors in Error Tracking

## Implementation details

> before

```
public void Remove(string name)
{
    var entry = _headers.Get(name);
    if (entry is not null)
    {
        _headers.Remove(name);
    }
}
```

> after
```
public void Remove(string name)
{
    var entry = _headers.Get(name);
    if (entry is not null)
    {
        _headers.Remove(entry);
    }
}
```

## Test coverage

Updated the sample application to detect this and if it is detected it
adds a tag which causes the tests to fail (and they did fail before I
applied the fix).

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

Used #7961
`/analyze-error` command based on the reported error in Error Tracking.
Seemed to work okay, it identified the issue, follow up was able to get
a reproduction quickly, but it really struggled to get the tests
updated.

Helpfully, the command output a document that linked to and copied the
actual `Grpc.Core.Api` code that we hit 👍
Saves some time searching on GitHub ourselves I guess.

This is a bit of an edge case here so I don't think very wide spread.

Reproduced Error:

```
2025-12-18 10:03:42.678 -05:00 [ERR] Error creating inactive client span for GRPC call System.InvalidCastException: Unable to cast object of type 'System.String' to type 'Entry'.
   at Grpc_Core_Api__D754F35622E28BAD.Grpc_Core_Metadata.Datadog_Trace_ClrProfiler_AutoInstrumentation_Grpc_IMetadata_3.Remove(Object )
   at Datadog.Trace.ClrProfiler.AutoInstrumentation.Grpc.MetadataHeadersCollection.Remove(String name)
   at Datadog.Trace.ClrProfiler.AutoInstrumentation.Grpc.MetadataHeadersCollection.Set(String name, String value)
   at Datadog.Trace.Headers.HeadersCollectionAccesor`1.Set(TCarrier carrier, String key, String value)
   at Datadog.Trace.Propagators.W3CTraceContextPropagator.Inject[TCarrier,TCarrierSetter](PropagationContext context, TCarrier carrier, TCarrierSetter carrierSetter)
   at Datadog.Trace.Propagators.SpanContextPropagator.Inject[TCarrier,TCarrierSetter](PropagationContext context, TCarrier carrier, TCarrierSetter carrierSetter)
   at Datadog.Trace.Propagators.SpanContextPropagator.Inject[TCarrier](PropagationContext context, TCarrier headers)
   at Datadog.Trace.ClrProfiler.AutoInstrumentation.Grpc.GrpcLegacy.Client.GrpcLegacyClientCommon.InjectHeaders[TMethod,TCallOptions](Tracer tracer, TMethod method, TCallOptions& callOptionsInstance)
 { MachineName: ".", Process: "[84684 Samples.GrpcLegacy]", AppDomain: "[1 Samples.GrpcLegacy.exe]", TracerVersion: "3.34.0.0" }

```
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---------

Co-authored-by: Andrew Lock <andrew.lock@datadoghq.com>