[http-client-csharp] Optional content type is incorrectly wrapped into content type enum

[ArcturusZhang]

Follow up of #9494 and Azure/azure-sdk-for-net#55300

The fix of above two fixes are focusing on checking if the parameter is "header".
But the real issue happens in the parameters of inputServiceMethod and the parameters in this list are method ("kind": "method") instead of "kind": "header"`. This part is still not fixed.

[ArcturusZhang]

Unless an operation defines its own content-type, TCGC will add this parameter to the method and parameter (and link them).
I debugged for multiple times, in majority of cases, the added method parameter and the operation parameter will share an instance of constant. And in this case, our previous fixes will work - we have a cache, if a TCGC type has been handled, next time, if we want to convert it via fromSdkType, we will return the result from the cache directly.

But in a very rare case (happened in a complicated template in mgmt plane), the type of method parameter content-type and operation parameter content-type do not share the same instance, and it causes our current issue.

TCGC owner (@tadelesh ) said there is no guarantee that constant types would be the same instance even if they have the same value, therefore we should not rely on this fact in the first place.

[ArcturusZhang]

Also I think our current behavior of wrapping constant is very cumbersome in the emitter.
We could have made it simpler - this could happen on the C# side instead of the emitter.
Emitter just outputs everything as is that it is an enum or a constant, and our TypeFactory could do something when it is a constant, and generate it as an enum type.