fix(client): properly cancel contexts in proposal pagination

[d0tsh]

Previously, context cancellation was deferred until function return potentially leading to resource leakage during pagination. Now each context is cancelled immediately after its request completes.

Changed in both v1 and v1beta1 proposal clients:

• Move context cancellation after each request
• Remove deferred cancellation
• Maintain same pagination and error handling logic

[mkaczanowski]

lgtm, thanks!