fix: preserve early workflow events in streams transport

[Zhengcy05]

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Summary

Fixes #40948.

Redis Streams subscriptions currently start from $, while streaming workflow tasks are enqueued before the listener performs its first XREAD. A fast worker can therefore publish workflow_started and early node events before $ is resolved, causing those events to be skipped even though they remain stored in Redis.

This PR captures the stream tip before dispatching a fresh workflow task and uses it as that subscription's starting cursor. Subscriptions without an explicit cursor continue to default to $, preserving the live-only behavior introduced to avoid replaying stale workflow_paused events in #34040.

Changes include:

• Add a cursor-capable topic protocol and per-subscription starting cursors for Redis Streams.
• Treat a missing or empty stream as cursor 0-0, since workflow streams are created lazily by the first XADD.
• Capture the stream tip before Celery task dispatch for fresh Workflow and Advanced Chat runs.
• Pass the captured topic and cursor through the event generator to the final Streams subscription.
• Correct the Streams test double so $ only observes entries appended after the initial read.
• Add regression coverage for explicit cursors while retaining coverage for the default $ behavior.

Validation:

• uv run --project api pytest -q api/tests/unit_tests/libs/broadcast_channel/redis/test_streams_channel_unit_tests.py api/tests/unit_tests/services/test_app_generate_service_streaming_integration.py api/tests/unit_tests/services/test_app_generate_service.py — 71 passed.
• Focused Ruff checks passed for all changed files.
• Ruff format checks passed for all changed files.
• Focused Pyrefly checks passed for all changed production files.
• Docker-backed integration tests were not run locally because they are CI-only.

Screenshots

N/A — backend event-delivery fix.

Checklist

• This change requires a documentation update, included: [Dify Document](https://github.com/langgenius/dify-docs)
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and vp staged (frontend) to appease the lint gods