docs: document exceptions raised by request()

[DORI2001]

Closes #6653

Summary

Adds :raises: RST docstring entries to the request() function documenting the exceptions that callers should be prepared to handle.

The documentation is added only on the main request() function (not repeated on each convenience method like get(), post(), etc.) to avoid duplication — all convenience methods delegate to request() and can raise the same exceptions.

Exceptions documented

Exception	When raised
ConnectionError	Network problem (DNS failure, refused connection, etc.)
Timeout	Request timed out (wraps ConnectTimeout and ReadTimeout)
TooManyRedirects	Configured redirect limit exceeded
URLRequired	No URL provided
MissingSchema	URL has no scheme (e.g. missing http://)
InvalidSchema	URL scheme is invalid or unsupported
InvalidURL	URL is otherwise malformed
InvalidHeader	A header value is invalid

These match the exceptions listed in the Requests exceptions API docs that are relevant to the request() call itself (as opposed to exceptions from response processing like JSONDecodeError).

[DORI2001]

Hi! Just checking in on this PR — happy to make any changes if needed. Thanks!