## Problem

When creating a new agent thread from the sidebar's **+** button (or via
the `NewThread` / `NewThreadInGroup` actions), the panel always returned
the cached draft thread — even if the user had since switched to a
different agent. This meant the new thread would use the wrong agent
(typically the default Zed Agent instead of the last-used external
agent).

The root cause was in `ensure_draft()`: it cached the draft and returned
it unconditionally on subsequent calls without checking whether its
agent
matched the current `selected_agent`.

Meanwhile, **cmd-n from within the agent panel** dispatched
`NewExternalAgentThread`, which calls `create_thread()` — this always
creates a fresh thread with the correct `selected_agent`, so it worked
fine.

## Fix

`ensure_draft()` now compares the cached draft's agent against the
desired agent. When they differ and the draft's editor is empty, the
stale draft is discarded and a new one is created with the correct
agent.
If the draft has user-typed content, it is preserved to avoid losing
work.

## Test

Added `test_draft_replaced_when_selected_agent_changes` which verifies:
1. A draft created with `NativeAgent` gets replaced when
`selected_agent`
   changes to a custom agent
2. The replacement draft uses the correct agent
3. Calling `activate_draft` again with the same agent reuses the cached
   draft (no unnecessary replacement)

The test fails without the fix (returns the stale `NativeAgent` draft)
and passes with it.

Release Notes:

- N/A