agent: Treat empty spawn_agent session IDs as new sessions

[haotian467]

Objective

spawn_agent uses an optional session_id to distinguish between creating a new session and resuming an existing one:

• None creates a new session.
• A session ID resumes an existing session.

In some model tool-call paths, a JSON null session ID is serialized as an empty string. Because an empty string still deserializes as Some(SessionId("")), spawn_agent attempts to resume that session and fails with:

No subagent session found with id


## Solution

Normalize an empty `session_id` to `None` while deserializing `SpawnAgentToolInput`.

This defensively handles the coerced representation while preserving the existing behavior for non-empty session IDs.

## Testing

Added `empty_session_id_deserializes_as_none`.
- Confirmed the regression test fails without the fix with:
  - `left: Some(SessionId(""))`
  - `right: None`
- Confirmed the regression test passes with the fix.
- Verified the existing sub-agent create, resume, cancellation, and output tests pass:

```sh
cargo test \
  -p agent \
  -p gpui_platform \
  --features gpui_platform/runtime_shaders \
  test_subagent_tool \
  --lib

• Ran cargo fmt --all -- --check.
• Ran git diff --check.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

---

Release Notes:

• Fixed spawn_agent failing to create a new sub-agent session when its session ID was serialized as an empty string.

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[haotian467]

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[bennetbo]

This was added in #60893