/conversations interface adjustment

### Self Checks

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### Dify version

1.11.2

### Cloud or Self Hosted

Self Hosted (Source)

### Steps to reproduce

The user parameter is mandatory for this API. However, I want to obtain the list of sessions created by all users in a specified period (that is, the user parameter is optional, and the start time and end time are two optional query fields). How can I minimize the modification of this API or add a new API?

### ✔️ Expected Behavior

a new API

### ❌ Actual Behavior

_No response_

The issue names only the /conversations API and no files or tests. Start by locating that endpoint and reviewing how its user, start-time, and end-time parameters are handled. Done means a new or minimally modified API can list sessions across users for an optional time period, with its behavior covered by the project’s API tests.

Written by the indexing model from the issue text.