fix: do not emit RF001 for methods from plain non-refit base interfaces

[reabr]

Fixes #2050

Problem

When a Refit interface inherited from a plain non-Refit interface (e.g. IBaseApi
with GetBaseUri()), the source generator would incorrectly emit RF001 diagnostics
for the base interface methods and attempt to generate stubs for them.

Solution

In Parser.cs, added a filter on derivedNonRefitMethods to only flag methods
from base interfaces that themselves contain at least one Refit HTTP method attribute.
Methods from plain non-Refit interfaces are now silently ignored since they are not
HTTP endpoints.

Testing

Added a fixture IInterfaceInheritingNonRefit.cs with a plain base interface and
a Refit interface inheriting from it. The snapshot test confirms that:

• No RF001 diagnostic is emitted for GetBaseUri()
• Only the Refit method GetUsers() gets a generated stub