Set content length for empty stream uploads

[Sean-Kenneth-Doherty]

Summary

• distinguish known zero-length streams from streams with unknown length
• send Content-Length: 0 for empty file-like uploads instead of Transfer-Encoding: chunked
• preserve chunked uploads for generators and file descriptors whose length cannot be determined

Fixes #6294.

Testing

• uv run --group test pytest tests/test_requests.py -k "empty_stream_with_auth or empty_file_stream or explicit_zero_content_length_stream or unknown_length_file_stream or chunked_upload_does_not_set_content_length_header or stream_with_auth_does_not_set_transfer_encoding_header"
• uv run --group test pytest tests/test_utils.py -k super_len
• uv run --group test pytest tests/test_lowlevel.py
• uv run --group typecheck pyright src/requests/models.py src/requests/utils.py
• uv run --with ruff ruff check src/requests/models.py src/requests/utils.py tests/test_requests.py
• uv run --with ruff ruff format --check src/requests/models.py src/requests/utils.py tests/test_requests.py
• uv run python -m compileall -q src/requests tests/test_requests.py tests/test_utils.py tests/test_lowlevel.py
• uv run --no-sync pytest tests/test_requests.py