CA1062 Not Reported for Receiver Parameter in Extension Block Syntax

### Describe the bug

CA1062 ("Validate arguments of public methods") fires for the receiver parameter of a classic extension method, but does **not** fire for the equivalent receiver in the new extension block syntax, even though both dereference the same value.

### Steps to reproduce

Compile the two equivalent snippets below with CA1062 enabled.

1. Classic extension method

```csharp
public static class StringExtensions
{
public static string TrimSlash(this string input)
{
return input.Trim('/'); // CA1062
}
}
```

2. Extension block

```csharp
public static class StringExtensions
{
extension(string input)
{
public string TrimSlash()
{
return input.Trim('/'); // No CA1062
}
}
}
```

### Expected behavior

Both forms expose a publicly reachable method that dereferences `input` without a null check. CA1062 should be reported consistently for both, or the reasoning for the difference should be documented.

### Actual behavior

CA1062 is reported only for the classic extension method. The extension block receiver is not analyzed, so the same potential null dereference goes unflagged.

### Is this a regression?

_No response_

### Are there any workarounds?

_No response_

### dotnet --info output

```console
.NET SDK:
Version: 10.0.201
Commit: 4d3023de60
Workload version: 10.0.200-manifests.0793c108
MSBuild version: 18.3.0-release-26153-122+4d3023de6

Runtime Environment:
OS Name: Windows
OS Version: 10.0.26100
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\10.0.201\

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
Version: 10.0.8
Architecture: x64
Commit: 94ea82652c

.NET SDKs installed:
8.0.101 [C:\Program Files\dotnet\sdk]
8.0.404 [C:\Program Files\dotnet\sdk]
9.0.305 [C:\Program Files\dotnet\sdk]
10.0.201 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 7.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 7.0.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
None

Environment variables:
DOTNET_CLI_TELEMETRY_OPTOUT [true]
DOTNET_CLI_UI_LANGUAGE [en-us]

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download
```

### IDE version

VS Code 1.128.0 (user setup) with C# Dev Kit 3.20.199

### Other details

_No response_

Start by compiling the two C# snippets in the issue with CA1062 enabled and compare the diagnostics for the classic extension receiver and extension block receiver. Done means CA1062 reports the equivalent null-dereference risk consistently, or the difference is documented with its rationale.

Written by the indexing model from the issue text.