fix: handle intrinsic functions in WebSocket API StageName property

[licjun]

Problem

When StageName is set to an intrinsic function (e.g. !Ref Param), SAM Translator crashes with:

TypeError: can only concatenate str (not "dict") to str

This happens in _construct_permission where self.stage_name is concatenated directly into a string to build the SourceArn.

Root Cause

Intrinsic functions like {"Ref": "Param"} are Python dicts. String concatenation with + fails when self.stage_name is a dict instead of a str. The validate_properties() bypass for intrinsics means the dict reaches the generator code without being caught.

Fix

Use isinstance(self.stage_name, str) to branch:

• Plain string: keep original behavior (no change to existing CloudFormation templates)
• Intrinsic: use fnSub variable substitution so CloudFormation resolves it at deploy time

if isinstance(self.stage_name, str):
    perms.SourceArn = fnSub(".../" + self.stage_name + "/...")
else:
    perms.SourceArn = fnSub(".../${__StageName__}/...", {"__StageName__": self.stage_name})

Also updated:

• property_types for StageName to accept one_of(IS_STR, IS_DICT) for better error messages (consistent with HttpApi)
• Type annotation to Intrinsicable[str] | None (consistent with HttpApi)
• stage_name parameter type in WebSocketApiGenerator.__init__ to Intrinsicable[str] | None

Testing

Added test_perms_with_intrinsic_stage_name unit test verifying the fix works for intrinsic StageName.