fix: handle FDv2 intentCode none as listening, reject unknown codes (#555)

Bring the FDv2 protocol handler's `server-intent` dispatch in line with
the spec and the Java implementation.

- `intentCode: none` keeps the handler in listening state so a
subsequent partial cycle works.
- Unrecognised intent codes return a protocol error instead of being
collapsed with `none`.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes streaming protocol state machine behavior for `server-intent`,
which can affect how clients process FDv2 payloads; scope is limited to
intent dispatch with new tests.
> 
> **Overview**
> Aligns **`server-intent`** handling in `FDv2ProtocolHandler` with the
FDv2 spec and Java client.
> 
> For **`intentCode: none`**, the handler still emits an immediate empty
**`kNone`** changeset, but it now leaves internal state in
**partial/listening** mode instead of **inactive**, so a later
**`put-object`** / **`payload-transferred`** cycle can produce a partial
changeset.
> 
> **Unrecognized** intent codes (deserialized as **`kUnknown`**) are no
longer treated like **`none`**: the handler **resets** and returns a
**`ProtocolError`** with *"server-intent had an unrecognized intent
code"*.
> 
> Tests cover a partial cycle after **`none`** and protocol errors for
unknown codes (e.g. **`future-code`**).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
f40b13f. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: beekld

libs/internal/src/fdv2_protocol_handler.cpp

@@ -73,11 +73,15 @@ FDv2ProtocolHandler::Result FDv2ProtocolHandler::HandleServerIntent(
         state_ = State::kFull;
     } else if (code == IntentCode::kTransferChanges) {
         state_ = State::kPartial;
-    } else {
-        // kNone or kUnknown: emit an empty changeset immediately.
-        state_ = State::kInactive;
+    } else if (code == IntentCode::kNone) {
+        state_ = State::kPartial;
         return data_model::FDv2ChangeSet{
             data_model::ChangeSetType::kNone, {}, data_model::Selector{}};
+    } else {
+        // kUnknown: an intent code we don't recognise.
+        Reset();
+        return Error::ProtocolError(
+            "server-intent had an unrecognized intent code");
     }
     return std::monostate{};
 }

libs/internal/tests/fdv2_protocol_handler_test.cpp

@@ -62,6 +62,37 @@ TEST(FDv2ProtocolHandlerTest, NoneIntentEmitsEmptyChangeSetImmediately) {
     EXPECT_FALSE(cs->selector.value.has_value());
 }
 
+TEST(FDv2ProtocolHandlerTest, NoneIntentAllowsSubsequentPartialCycle) {
+    FDv2ProtocolHandler handler;
+
+    handler.HandleEvent("server-intent", MakeServerIntent("none"));
+    handler.HandleEvent("put-object",
+                        MakePutObject("flag", "my-flag", kFlagJson));
+    auto result = handler.HandleEvent("payload-transferred",
+                                      MakePayloadTransferred("s1", 1));
+
+    auto* cs = std::get_if<data_model::FDv2ChangeSet>(&result);
+    ASSERT_NE(cs, nullptr);
+    EXPECT_EQ(cs->type, data_model::ChangeSetType::kPartial);
+    ASSERT_EQ(cs->changes.size(), 1u);
+    EXPECT_EQ(cs->changes[0].key, "my-flag");
+}
+
+// ============================================================================
+// Unknown intent
+// ============================================================================
+
+TEST(FDv2ProtocolHandlerTest, UnknownIntentReturnsProtocolError) {
+    FDv2ProtocolHandler handler;
+
+    auto result =
+        handler.HandleEvent("server-intent", MakeServerIntent("future-code"));
+
+    auto* err = std::get_if<FDv2ProtocolHandler::Error>(&result);
+    ASSERT_NE(err, nullptr);
+    EXPECT_EQ(err->kind, FDv2ProtocolHandler::Error::Kind::kProtocolError);
+}
+
 // ============================================================================
 // kTransferFull intent
 // ============================================================================