fix #25976: treat proc-type forbids as an empty tag set (#25980)

fix #25976

Initialize tagEffects for proc types that declare .forbids but omit
.tags,
so they behave like explicit tags: [] during indirect-call effect
tracking.
Add a regression for the nested callback assignment case.

(cherry picked from commit 4b1444e728)

tests/effects/tissue25976.nim

@@ -0,0 +1,28 @@
+type
+  NestedPoll = object of RootEffect
+
+  CallbackFunc = proc(arg: pointer) {.gcsafe, raises: [], forbids: [NestedPoll].}
+  TaggedCallbackFunc = proc(arg: pointer) {.gcsafe, raises: [], tags: [], forbids: [NestedPoll].}
+
+  InternalAsyncCallback = object
+    fn: CallbackFunc
+
+  TaggedInternalAsyncCallback = object
+    fn: TaggedCallbackFunc
+
+proc closeSocket(aftercb: CallbackFunc = nil) =
+  proc continuation(udata: pointer) =
+    aftercb(nil)
+
+  let acb = InternalAsyncCallback(fn: continuation)
+  discard acb
+
+proc closeSocketTagged(aftercb: TaggedCallbackFunc = nil) =
+  proc continuation(udata: pointer) =
+    aftercb(nil)
+
+  let acb = TaggedInternalAsyncCallback(fn: continuation)
+  discard acb
+
+closeSocket()
+closeSocketTagged()