C#: Fix false positives in cs/log-forging for extension methods

[Gregro]

The log forging query previously treated all arguments to methods called on logger types as sinks, including user-defined extension methods. This caused false positives when extension methods sanitize input internally, since interprocedural analysis was bypassed.

Now, only direct instance method calls on logger types are treated as sinks. User-defined extension methods are analyzed interprocedurally, allowing the query to see sanitization within method bodies.

Known framework extension methods (Microsoft.Extensions.Logging.LoggerExtensions) are modeled as explicit sinks via Models as Data.

Fixes #15824