Add the Activity, SubscriptionStep and MessageResponse models

These are the types the per-resource extras and the activity endpoints
need, split out so they land before the code and tests that use them.

`MessageResponse` covers the `{"message": "..."}` body that the action
endpoints answer with, defaulting `message` so a reply carrying none still
decodes. `int_or_numeric_string` goes in alongside them: the spec calls
`frequency_id` a number, but a real 5.13.50 server sends the string `"5"`,
and a strict `i64` turned that into a hard decode error *after* the record
had already been created.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/models/activity.rs

@@ -0,0 +1,122 @@
+use serde::{Deserialize, Serialize};
+use serde_json::{Map, Value};
+
+/// One row of the activity log, as `GET /activities` returns it.
+///
+/// Every reference is a hashed id, or an empty string when the activity does
+/// not concern that kind of record. `activity_type_id` arrives as a string
+/// here but as a number in the [feed](ActivityFeedEntry).
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct Activity {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub id: Option<String>,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub activity_type_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub user_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub company_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub client_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub contact_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub vendor_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub vendor_contact_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub invoice_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub quote_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub credit_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub payment_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub recurring_invoice_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub recurring_expense_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub purchase_order_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub expense_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub task_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub token_id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub is_system: bool,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub notes: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub ip: String,
+    #[serde(default)]
+    pub created_at: Option<i64>,
+    #[serde(default)]
+    pub updated_at: Option<i64>,
+
+    #[serde(flatten)]
+    pub extra: Map<String, Value>,
+}
+
+/// A named record an activity refers to: the display label and its hashed id.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct ActivityRef {
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub label: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub hashed_id: String,
+
+    #[serde(flatten)]
+    pub extra: Map<String, Value>,
+}
+
+/// One entry of the activity feed (`activity_feed`, `entity_activity`).
+///
+/// A different shape from [`Activity`]: the server resolves the records an
+/// activity mentions into labelled references. Which references are present
+/// depends on the activity type, so each is optional, and any the server adds
+/// that this does not name lands in `extra`.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct ActivityFeedEntry {
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub id: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub hashed_id: String,
+    #[serde(default, deserialize_with = "super::common::int_or_numeric_string")]
+    pub activity_type_id: i64,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub notes: String,
+    #[serde(default, deserialize_with = "super::common::null_to_default")]
+    pub ip: String,
+    #[serde(default, deserialize_with = "super::common::int_or_numeric_string")]
+    pub created_at: i64,
+
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub user: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub client: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub vendor: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub invoice: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub quote: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub credit: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub payment: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub task: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub expense: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub purchase_order: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub recurring_invoice: Option<ActivityRef>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub recurring_expense: Option<ActivityRef>,
+
+    #[serde(flatten)]
+    pub extra: Map<String, Value>,
+}