Fix Client::list_sessions wire shape — wrap filter under params.filter

The hand-authored `Client::list_sessions` was serializing the optional
`SessionListFilter` directly onto the JSON-RPC `params` object,
flattening fields like `repository` / `branch` / `cwd` / `gitRoot` to
the top level. The `session.list` request shape that the runtime accepts
puts the filter under `params.filter` — and that's what every other SDK
sends:

- Node `nodejs/src/client.ts:1178-1180`: `sendRequest("session.list", { filter })`
- Go `go/types.go`: `listSessionsRequest { Filter *SessionListFilter }`
- Python `python/copilot/client.py:1907-1911`: `payload["filter"] = ...`
- .NET `dotnet/src/Client.cs`: `record ListSessionsRequest(SessionListFilter? Filter)`

Because the runtime silently ignores unknown top-level keys on
`session.list`, calling `list_sessions(Some(filter))` was functionally
equivalent to `list_sessions(None)` in 0.0.x — every filter field was
discarded by the runtime, returning an unfiltered session list. No
runtime error, no log, just silently broken. Functionally dead on the
wire, same class as the elicitation `requestedSchema` fix in `c58e2f2`.

The mock-server test `list_sessions_serializes_typed_filter` asserted on
the flat shape it observed (`request["params"]["repository"]`) rather
than the schema's wrapped shape, so the bug round-tripped through both
ends — the implementation produced the wrong shape, the test verified
the wrong shape. Same root cause as the elicitation test gap.

Fix:

- `Client::list_sessions` now wraps the filter: `Some(f) ->
  serde_json::json!({ "filter": f })`, `None -> serde_json::json!({})`.
  `None` omits the filter key entirely (matches Go's `omitempty`
  behavior; Node's `{ filter: undefined }` also omits via JSON-stringify).
- Mock-server test now asserts on the wrapped path
  (`params.filter.repository`, `params.filter.branch`) AND explicitly
  asserts the flattened fallback is gone (`params.get("repository")`
  must return `None`). Same regression-prevention pattern as the
  elicitation fix at `session_test.rs:1248-1251`.
- CHANGELOG entry under `### Fixed` documenting the wire-shape fix and
  the test gap that masked it.

Validation:

- 209 tests pass (no count change — same test, stricter assertions).
- `cargo doc -D warnings` clean.
- `cargo +nightly-2026-04-14 fmt --check` clean.
- `cargo clippy --all-features --all-targets -- -D warnings` clean.

Caught by the gap-analysis structural-correctness pass walking every
hand-authored `client.call("...")` site against the schema and the four
other SDKs. This is the second wire-shape bug found by that pass; the
first was the `SessionUi::elicitation` `schema` -> `requestedSchema`
fix in `c58e2f2`. The audit confirms `session.list` is the only other
new bug — three Rust-unique surfaces (`session.respondToUserInput`,
`session.sendTelemetry`, top-level `sendTelemetry` /
`server.sendTelemetry`) are uncheckable cross-SDK and queued as
post-0.1.0 runtime-acceptance verification.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tclem

rust/CHANGELOG.md

@@ -341,6 +341,17 @@ public surface.
   round-tripped through the same misnamed field, so the bug slipped past
   unit tests; the test now asserts on `requestedSchema` and explicitly
   rejects a stray `schema` key.
+- `Client::list_sessions` now wraps the optional filter under `params.filter`
+  on the wire, matching the `session.list` request shape that Node, Python,
+  Go, and .NET ship. The hand-authored implementation was flattening the
+  filter fields directly onto `params`, which the runtime silently ignored
+  — so `list_sessions(Some(filter))` was functionally equivalent to
+  `list_sessions(None)` in 0.0.x. Same class of bug as the elicitation
+  wire fix above: the existing mock-server test asserted on the flat shape
+  it observed rather than the schema's wrapped shape, so the bug
+  round-tripped through both ends. The test now asserts the wrapped path
+  (`params.filter.repository`) and explicitly rejects the flattened
+  fallback (`params.repository`).
 
 ### Notes
 - Minimum supported Rust version (MSRV): 1.94.0 (pinned via

rust/src/lib.rs

@@ -1199,7 +1199,7 @@ impl Client {
         filter: Option<SessionListFilter>,
     ) -> Result<Vec<SessionMetadata>, Error> {
         let params = match filter {
-            Some(f) => serde_json::to_value(f)?,
+            Some(f) => serde_json::json!({ "filter": f }),
             None => serde_json::json!({}),
         };
         let result = self.call("session.list", Some(params)).await?;

rust/tests/session_test.rs

@@ -679,11 +679,18 @@ async fn list_sessions_serializes_typed_filter() {
 
     let request = read_framed(&mut server_read).await;
     assert_eq!(request["method"], "session.list");
-    assert_eq!(request["params"]["repository"], "octocat/hello");
-    assert_eq!(request["params"]["branch"], "main");
-    // cwd / gitRoot are None and must be omitted from the wire payload.
-    assert!(request["params"].get("cwd").is_none());
-    assert!(request["params"].get("gitRoot").is_none());
+    assert_eq!(request["params"]["filter"]["repository"], "octocat/hello");
+    assert_eq!(request["params"]["filter"]["branch"], "main");
+    // cwd / gitRoot are None and must be omitted from the filter object.
+    assert!(request["params"]["filter"].get("cwd").is_none());
+    assert!(request["params"]["filter"].get("gitRoot").is_none());
+    // Regression check: filter must be wrapped under `params.filter`, not
+    // flattened onto `params` directly. All other SDKs (Node/Python/Go/.NET)
+    // wrap; flattening is silently ignored by the runtime.
+    assert!(
+        request["params"].get("repository").is_none(),
+        "wire shape is `params.filter.*`, not `params.*` — see Node/Go/Python/.NET"
+    );
 
     let id = request["id"].as_u64().unwrap();
     let response = serde_json::json!({