Expose JSON type converter unhandled sentinel type

[Lingavasan]

Summary

• expose a public JSONTypeConverterUnhandled type for JSONTypeConverter.Unhandled
• keep the old private _JSONTypeConverterUnhandled name as an internal compatibility alias
• update the README and converter tests to use the public return type

Fixes #1434

Validation

• .venv/bin/ruff format --check temporalio/converter/_payload_converter.py temporalio/converter/init.py tests/test_converter.py
• .venv/bin/ruff check temporalio/converter/_payload_converter.py temporalio/converter/init.py tests/test_converter.py
• git diff --check
• .venv/bin/python -m py_compile temporalio/converter/_payload_converter.py temporalio/converter/init.py tests/test_converter.py
• direct source validation for public JSONTypeConverterUnhandled import and to_typed_value return annotation
• direct source validation for a custom JSONTypeConverter with value_to_type

Note: I attempted the focused pytest coverage, but this local checkout cannot run repo pytest without the Temporal Rust bridge/test harness being built. The checkout has an empty sdk-core submodule and this machine does not have cargo available, so CI is the authoritative full test run.

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