Handle null upload target fields (#10959)

## Description
Use `serde_with::DefaultOnNull` on `UploadTarget::fields` so the Rust
client treats an explicit JSON `null` the same as a missing field and
deserializes it as an empty vector. This keeps local-to-cloud handoff
snapshot uploads from failing when the server returns `"fields": null`.

## Linked Issue
N/A - Slack request from `#pod-oz-handoff`.

- [ ] The linked issue is labeled `ready-to-spec` or
`ready-to-implement`.
- [ ] Where appropriate, screenshots or a short video of the
implementation are included below (especially for user-visible or UI
changes).

## Testing
- `cargo fmt --manifest-path /workspace/warp/app/Cargo.toml --check`
- `CARGO_BUILD_JOBS=1 cargo nextest run --manifest-path
/workspace/warp/Cargo.toml -p warp -E
'test(upload_target_deserializes_null_fields_as_empty)'`
- `CARGO_BUILD_JOBS=1 cargo check --manifest-path
/workspace/warp/Cargo.toml -p warp --lib`

- [ ] I have manually tested my changes locally with `./script/run`

### Screenshots / Videos
N/A - no UI changes.

## Agent Mode
- [x] Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-OZ: Fixed local-to-cloud handoff snapshot upload allocation
when the server returns no upload form fields.

_Conversation:
https://staging.warp.dev/conversation/d49e5635-e39d-46d4-b6fd-12f04271a381_
_Run:
https://oz.staging.warp.dev/runs/019e2875-6204-77b7-9d97-70b6a9e871d8_
_This PR was generated with [Oz](https://warp.dev/oz)._

Co-authored-by: Oz <oz-agent@warp.dev>

Author: bnavetta

app/src/server/server_api/harness_support.rs

@@ -21,13 +21,15 @@ pub use super::presigned_upload::FileUploadBody;
 pub use super::presigned_upload::UploadBody;
 
 /// A presigned upload target returned by the server.
+#[serde_with::serde_as]
 #[derive(Debug, Clone, serde::Deserialize)]
 pub struct UploadTarget {
     pub url: String,
     pub method: String,
     pub headers: HashMap<String, String>,
     /// Ordered multipart form fields for POST uploads.
     #[serde(default)]
+    #[serde_as(deserialize_as = "serde_with::DefaultOnNull")]
     pub fields: Vec<UploadField>,
 }
 

app/src/server/server_api/harness_support_tests.rs

@@ -1,5 +1,20 @@
 use crate::ai::artifacts::Artifact;
 
+#[test]
+fn upload_target_deserializes_null_fields_as_empty() {
+    use super::UploadTarget;
+
+    let target: UploadTarget = serde_json::from_value(serde_json::json!({
+        "url": "https://example.com/upload",
+        "method": "PUT",
+        "headers": {},
+        "fields": null
+    }))
+    .unwrap();
+
+    assert_eq!(target.fields.len(), 0);
+}
+
 /// Assert that `Artifact`s serialize to the expected format for the /harness-support/report-artifact
 /// endpoint.
 /// If `Artifact` serialization changes, this test will catch it.