CBG-5792: Wait for the changes feed before starting a replicator

[torcolvin]

CBG-5792: Wait for the changes feed before starting a replicator

Fix test_pull_resurrected_doc flakes:

test_pull_resurrected_doc failed because an on demand import does not prime the caching feed before a one shot replication

• Add an opt-in wait_for_caching_feed to the Sync Gateway write and read helpers, factored out of the readback update_document already did.
• Update call sites that require wait_for_caching_feed that do not rely on import, which will be fixed separately.

[torcolvin]

@bbrks I'm interested in your feedback if there's a way we can fold this into a framework in a better way.

I propose committing this temporarily to hide this flake but then come up with better strategies for avoiding the flakes in the first place.

[Copilot]

🟢 Approval recommended

The changes are additive/opt-in and consistently applied at relevant call sites, with only minor doc/test-step wording nits noted.

Pull request overview

This PR hardens replication tests against Sync Gateway channel-cache timing by optionally waiting for written/imported revisions to appear in a request_plus _changes feed before starting a replicator, reducing flakes like test_pull_resurrected_doc.

Changes:

• Added wait_for_caching_feed opt-in behavior to Sync Gateway helpers (update_documents, delete_document, get_document) and factored the “read-back from changes feed” logic into a shared helper.
• Added SyncGatewayCluster.wait_for_sequence(...) to ensure all SGW nodes’ channel caches have caught up to a known sequence.
• Updated several QE/dev_e2e tests to use wait_for_caching_feed=True at write/read points where immediate replication previously raced the cache/import pipeline.

File summaries

File	Description
tests/QE/test_replication_upgrade_delta_sync.py	Uses wait_for_caching_feed=True after SGW mutation to avoid cache-race before subsequent replication/validation.
tests/QE/test_replication_functional.py	Waits for the caching feed after initial document creation to prevent role/channel replication races.
tests/dev_e2e/test_replication_filter.py	Waits for the caching feed after creating a public-channel doc so the pull replicator sees it deterministically.
tests/dev_e2e/test_replication_behavior.py	Reworks flake-prone steps to wait for cache/import visibility (including cluster-wide sequence catch-up) before replication.
tests/dev_e2e/test_replication_auto_purge.py	Waits for caching feed after document creation to avoid timing-dependent replication behavior.
client/src/cbltest/api/syncgatewaycluster.py	Adds a cluster-level helper to wait for a specific sequence across all SGW nodes.
client/src/cbltest/api/syncgateway.py	Adds since support to get_changes, introduces _wait_for_caching_feed, and extends write/read helpers with wait_for_caching_feed.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 2
• Review effort level: Lite

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