PathLike upload tuples fail with an opaque connection error

Start with is_file_content() and _transform_file(), which handle the shared upload path for certificate, extension, and session uploads. Run the existing upload tests and add sync and async coverage for PathLike values in the supported tuple shapes. Done means tuple uploads read the path while preserving the custom filename, content type, and optional headers.

What happened?

A bare Path works as an upload, but the tuple form accepted by FileTypes does not:

from pathlib import Path

client.sessions.uploads.create(
    "session-id",
    file=("custom.md", Path("README.md"), "text/markdown"),
)

On current main at 5f5274b, this raises:

APIConnectionError: Connection error.

The underlying cause is:

AttributeError: 'PosixPath' object has no attribute 'read'

The same path is shared by certificate, extension, and session uploads.

Why it happens

is_file_content() treats every tuple as terminal file content. That means _transform_file() returns the tuple unchanged before its tuple-handling branch can read the PathLike value inside it. HTTPX later receives the raw Path and tries to call .read() on it.

Expected behavior

A PathLike inside any supported upload tuple should be read just like a bare Path, while preserving the custom filename, content type, and optional headers.

I have a small fix ready that keeps tuples separate from direct file content and adds sync and async coverage for all three supported tuple shapes.