Raise unclear error when creating Session with the same name as any `PENDING` session

### What Operating System(s) are you seeing this problem on?

Linux (aarch64)

### Backend.AI version

main

### Describe the bug

When you try to create a Session with the same name as any existing `PENDING` session, the error message pops up like below, which is unclear.

### To Reproduce

Let Session be hanging on `PENDING` status, I simply change code like below.

and create Sessions with the same name.

### Expected Behavior

Return a clear error message and pops the message like below.

### Anything else?

_No response_

JIRA Issue: BA-246

Start by reproducing session creation with two sessions sharing a name while one remains in PENDING, using the Linux/aarch64 setup described in the issue. Compare the current and expected error messages, then trace the session-creation entry point to identify where the unclear error is produced. Done means the duplicate-name case returns the clear expected message and relevant tests pass.

Written by the indexing model from the issue text.