feat(python): support timeout=None to disable timeout

[Copilot]

timeout=None on per-request methods fell back to the client-level default instead of disabling the timeout, making it impossible to issue truly unbounded requests and diverging from HTTPX conventions.

Core changes

• RequestOptions.timeout changed from Option<Duration> to Option<Option<Duration>>:
  • None → inherit client default (unchanged)
  • Some(None) → disable timeout entirely
  • Some(Some(d)) → specific override
• impit.rs: maps Some(None) → Duration::MAX before handing off to reqwest (the only per-request escape hatch reqwest exposes without rebuilding the client)
• impit-node: trivial one-liner wrap of the existing value in Some(…)

Python binding changes

Distinguishing "not passed" from "explicitly None" requires a sentinel because PyO3 maps both to Rust None for Option<f64>. Solution:

• Per-request timeout parameter changed from Option<f64> to Py<PyAny> with an interned string sentinel ("__impit_use_client_default__") as the PyO3 default
• timeout_from_pyobj() parses the three states; default_timeout() returns the sentinel for use in #[pyo3(signature = (...))]
• USE_CLIENT_DEFAULT is exposed as a public module constant (matching HTTPX's convention) so callers can pass it explicitly

Usage

from impit import Client, USE_CLIENT_DEFAULT

client = Client(timeout=5.0)

client.get(url)                          # uses 5s client default
client.get(url, timeout=USE_CLIENT_DEFAULT)  # same — explicit form
client.get(url, timeout=None)            # no timeout (wait indefinitely)
client.get(url, timeout=30.0)            # 30s override

Type stubs and docstrings updated to reflect the new timeout: float | None = USE_CLIENT_DEFAULT signature on all per-request methods.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Support None as timeout to disable timeout entirely</issue_title>
<issue_description>Currently, passing timeout=None to Client.request() / AsyncClient.request() falls back to the client-level default (30s) because the Rust side skips client_request.timeout() when the value is None:

// impit.rs:443-444
if let Some(timeout) = timeout {
    client_request = client_request.timeout(timeout);
}

HTTPX treats timeout=None as "no timeout" (wait indefinitely). Since impit's Python API aims to follow HTTPX conventions, it would be good to match this behavior.

Proposed: When timeout=None is passed per-request, actively disable the timeout instead of falling back to the client default.</issue_description>

<agent_instructions>Implement the changes in impit-python. Make minimal changes to achieve the goal. Do not add excessive comments. Make sure to describe the new behaviour in the docblock (or generated documentation). Use conventional commits for the commit messages and PR titles. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Support None as timeout to disable timeout entirely #401

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[barjin]

This is rather messy and a candidate for refactoring, but I suppose it's as good as we can do without changing the API too much.