fix: generate enum fields as str/int so raw values pass type checkers

[Alan4506]

Description of changes:

When we generate a Python dataclass for a Smithy structure, each member that targets an enum shape gets a type annotation. Before #702, these fields were annotated as str | None (or int | None for intEnums), so users could pass either a raw string like "RETRIEVAL_MODE" or the enum constant ChatMode.RETRIEVAL_MODE: both worked and both passed type checkers.

#702 changed genericEnum in PythonSymbolProvider to return the enum class symbol directly, so those fields became ChatMode | None. This is more precise, but it means passing a raw string (awslabs/aws-sdk-python#54 for example) now fails static type checkers (e.g., ty) with errors like:

error[invalid-argument-type]: Argument is incorrect
  --> tests/integration/test_bidirectional_streaming.py:36:38
   |
36 |             value=ConfigurationEvent(chat_mode="RETRIEVAL_MODE")
   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ Expected `ChatMode | None`, found `Literal["RETRIEVAL_MODE"]`
   |

This PR restores the original str/int field typing while preserving all #702 functionality (error correction, unknown-value handling).

Testing:

Regenerated the qbusiness and connecthealth clients locally. Verfied their integration tests (awslabs/aws-sdk-python#54 and awslabs/aws-sdk-python#62) still passed, which means the #702 functionality is preserved. Type checker ty also passed with no issues.

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