fix(dispatch): stop misreading list/dict handler returns as raw bytes/wrong status

[ZhuchkaTriplesix]

Problem

map_handler_return() tried extract::<Vec<u8>>() before checking for a plain dict/list. pyo3 happily extracts any Python sequence of small ints as Vec<u8> — including an empty list — so:

• return [] sent an empty application/octet-stream body instead of JSON [].
• return [1, 2, 3] sent raw bytes \x01\x02\x03 instead of JSON [1,2,3].

Separately, any dict containing both a status and body key was unconditionally treated as a structured HTTP response — so ordinary JSON data shaped that way (e.g. {"status": 1, "body": "x"}) had its HTTP status silently set to 1 instead of being returned as data.

Fix

• Check for PyDict/PyList before the String/Vec<u8> extraction attempts, so JSON payloads always fall through to JSON serialization instead of being coerced to bytes.
• Require status to be a plausible HTTP status code (100-599, RFC 9110 §15) before treating a dict as a structured response — preserves the existing documented {"status": ..., "body": ...} shorthand (docs/usage.md, tests/test_exception_handlers.py) for real status codes, while letting ambiguous data round-trip as JSON.

Testing

New tests/test_handler_return_mapping.py:

• test_empty_list_is_json_not_octet_stream

• test_int_list_is_json_not_raw_bytes

• test_dict_with_out_of_range_status_roundtrips_as_data

• test_dict_with_valid_status_and_body_is_still_a_structured_response (confirms the documented shorthand still works)

• cargo build --lib / cargo clippy --all-targets: clean.

• cargo test --lib: 26 passed.

• Full suite: 168 passed (164 + 4 new), 1 skipped.

• ruff check / ruff format --check: clean.

Closes #211