python(fix): bound sync calls at the transport layer

[wei-qlu]

Summary

The sync timeout cap wrapped the whole coroutine, so it aborted long-running waiters (wait_until_complete, wait_and_download) at the cap even when they were polling fine or the caller passed a larger timeout_secs.

This removes the cap and bounds calls at the transport layer instead: gRPC keeps its 60s deadline, and REST gets a default 60s timeout it never had (an inter-byte read timeout, so stalled sockets fail fast but healthy long downloads don't). Waiters now honor their own timeout_secs.

REST requests now default to a 60s timeout, configurable via RestConfig(request_timeout=...) and overridable per call.

Test plan

• Unit tests (REST timeout default/override/disable, plus existing sync-wrapper
  and gRPC deadline tests).
• Manual: ran a data import that takes a few minutes and confirmed the sync
  wait_until_complete() runs to completion instead of being cut off at the old cap.

[github-actions]

Python docs preview: https://sift-stack.github.io/sift/python/pr-613/

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