[Caideyipi]

Description

PR #18738 lets consumers whose assigned regions are idle borrow ready events from regions assigned to other consumers. This can send events from one region to multiple consumers and reorder downstream processing in streaming window workloads.

Add the startup setting subscription_consensus_fallback_polling_enabled. It defaults to true to preserve existing behavior. Set it to false in DataNode configuration and restart the DataNodes to restrict polling to each consumer's assigned regions. Normal ownership reassignment remains unchanged.

Regression tests cover configuration loading, retaining the existing enabled fallback, disabling borrowing when assigned regions are empty, and disabling borrowing when a consumer owns no regions. A cluster IT uses one DataRegion and two consumers in the same group to verify that the unassigned consumer cannot borrow messages while the assigned owner still receives every row.

Validation

• Spotless formatting and IT selection checks passed.
• Clean targeted unit-test reactor run: 14 tests passed.
• Clean Chinese-locale reactor test compilation passed.
• Remote 1 ConfigNode / 3 DataNode cluster IT: 1 test passed, with no failures, errors, or skips. Subscription was temporarily enabled in an isolated verification source copy; the enablement override is excluded from this PR.
• Negative control: enabling fallback caused the same IT to fail at the assertion that the unassigned consumer must not receive messages.