Summary:
As of Postgres commit c7ff7663e47fc4e295700101912f2b7dd53c1f4b, subplans share their parent's `Estate`. Consequently, Postgres commit 520acab171244b55d816c70b9a89280b09937925 ensures that the scan direction (`es_direction`) is saved and restored before and after subplan execution.

A similar save-and-restore mechanism is required for `yb_exec_params` to prevent subplans from incorrectly sharing the value of `yb_exec_params.limit_count`. Without this safeguard, the issues described in the linked GitHub discussions can occur.

This patch addresses those issues by ensuring `yb_exec_params` is properly saved and restored across subplan executions.

Test Plan: ./yb_build.sh --java-test 'org.yb.pgsql.TestPgRegressThirdPartyExtensionsPgvector'

Reviewers: telgersma, kramanathan, #db-approvers

Reviewed By: telgersma

Subscribers: svc_phabricator, jason, yql

Differential Revision: https://phorge.dev.yugabyte.com/D42699