`TypeMapping.get()` method fails to retrieve system types when the type name contains `::`

Version and Platform (required):

• Binary Ninja Version: 5.1.7512-test
• Edition: Ultimate
• OS: MacOS
• OS Version:15.4.1
• CPU Architecture: M4 Max

Bug Description:
TypeMapping.get() method fails to retrieve system types when the type name contains :: (double colons), while direct dictionary access works correctly. User-defined types with :: in their names can be retrieved normally with get().

Steps To Reproduce:

1. Open any binary with C++ symbols that creates types with namespace qualifiers
2. Access the Python console
3. Verify type exists: type bv.types to search for example std::string exists
4. Try to get the type: bv.types.get('std::string') returns None
5. With non-namespaced type works correctly

Expected Behavior:
bv.types.get('std::string') should return the same result as bv.types['std::string'] for system types containing ::.

Screenshots/Video Recording:

>>> bv.types
<TypeMapping 132 symbols: {... , 'std::string': <type: immutable:StructureTypeClass 'class std::string'>, ...>
>>> bv.types.get("std::string")
>>> bv.types['std::string']
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Applications/Binary Ninja.app/Contents/MacOS/plugins/../../Resources/python/binaryninja/binaryview.py", line 2062, in __getitem__
    raise KeyError(key)
KeyError: 'std::string'

Start with binaryninja/binaryview.py around line 2062 and the TypeMapping.get entry point. Reproduce the Python console example using bv.types.get('std::string') and bv.types['std::string']; done when namespaced system types return the same result through both access paths.