WORKFLOW_MAX_EXECUTION_TIME no longer takes effect after upgrading to v1.16.0, workflow forcibly interrupted at 1200s

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### Dify version

v1.16.0

### Cloud or Self Hosted

Self Hosted (Docker)

### Steps to reproduce

### ✔️ Expected Behavior

The environment variable WORKFLOW_MAX_EXECUTION_TIME should be respected to control the maximum execution duration of workflows. For example, if set to 3600 seconds, workflows should be allowed to run up to that limit without being interrupted prematurely.

### ❌ Actual Behavior

After upgrading to v1.16.0, WORKFLOW_MAX_EXECUTION_TIME is ignored. Workflows are now forcibly interrupted exactly at 1200 seconds (20 minutes) regardless of the configured value. No alternative variable or configuration method has been found to extend this limit.

Start by searching the repository for WORKFLOW_MAX_EXECUTION_TIME and for the 1200-second limit, then trace how workflow execution limits are configured in the self-hosted Docker setup. Compare the v1.16.0 behavior with the expected configured value; done means a workflow can run up to the configured limit instead of being interrupted at 1200 seconds.

Written by the indexing model from the issue text.