feat: support timeouts in the send_request context helper

[Battleplus]

Closes #2138

Summary

SendRequestFunction (used via context.send_request() in request handlers) only accepted url, method, payload and headers, so a handler could not bound how long an extra HTTP call may take. All four HTTP client implementations (HttpxHttpClient, CurlImpersonateHttpClient, ImpitHttpClient, PlaywrightHttpClient) already support a per-request timeout: timedelta | None — this PR just exposes it through the public contract.

Changes

• src/crawlee/_types.py: add timeout: timedelta | None = None to the SendRequestFunction protocol + docstring.
• src/crawlee/crawlers/_basic/_basic_crawler.py: thread timeout through the _prepare_send_request_function closure into HttpClient.send_request.
• src/crawlee/crawlers/_playwright/_playwright_http_client.py: fix — PlaywrightHttpClient passed timeout.total_seconds() to Playwright's APIRequestContext.fetch, which expects milliseconds. Without this fix, any timeout passed via context.send_request() on a Playwright crawler would be ~1000x too short (e.g. timedelta(seconds=12) → 12ms).
• tests/unit/crawlers/_basic/test_basic_crawler.py: regression test — context.send_request('/slow?delay=2', timeout=timedelta(milliseconds=100)) raises asyncio.TimeoutError; plus a unit test asserting timeout is forwarded to HttpClient.send_request.
• tests/unit/crawlers/_playwright/test_playwright_http_client.py: regression test asserting the seconds→milliseconds conversion.

Validation

• pytest tests/unit/crawlers/_basic/test_basic_crawler.py → 112 passed, 1 skipped
• New tests (forwarding, real timeout, Playwright ms conversion) all pass
• ruff check + ruff format --check clean on all touched files

Note: no new dependency; timeout uses the existing timedelta convention across the Python HTTP clients (the JS implementation names it timeoutMillis, but the Python clients are timedelta-based).