fix: handle END in conditional edge path_map lookup

[Chase-Xuu]

Summary

When a conditional router returns END ('__end__') but path_map does not include an explicit END mapping, the code raises KeyError('__end__').

Fix

This PR treats END as a special case in _branch.py's _finish method - it now passes through directly without requiring a path_map entry, matching user expectations.

The Change

# Before
destinations = [r if isinstance(r, Send) else self.ends[r] for r in result]

# After  
destinations = [r if isinstance(r, Send) else (r if r == END else self.ends[r]) for r in result]

Fixes #6770