refactor: fix remaining _id→id in tests, mocks, and documentation

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot

content/docs/protocol/objectql/schema.mdx

@@ -303,7 +303,7 @@ Every object automatically includes system fields:
 
 ```yaml
 # Auto-generated (not defined in .object.yml)
-_id:
+id:
   type: id
   label: Record ID
   read_only: true
@@ -637,7 +637,7 @@ parent_id:
 Query polymorphic fields:
 ```typescript
 const activity = await ObjectQL.findOne('activity', id);
-// activity.parent_id = { _id: '123', _type: 'account' }
+// activity.parent_id = { id: '123', _type: 'account' }
 ```
 
 ### Computed Fields (Virtual)

content/docs/protocol/objectql/security.mdx

@@ -256,7 +256,7 @@ const user = await ObjectQL.findOne('user', '123');
 
 // Result (salary/ssn stripped):
 // {
-//   _id: '123',
+//   id: '123',
 //   name: 'John Doe',
 //   email: 'john@example.com'
 //   // salary: REMOVED
@@ -268,7 +268,7 @@ const user = await ObjectQL.findOne('user', '123');
 
 // Result (salary visible):
 // {
-//   _id: '123',
+//   id: '123',
 //   name: 'John Doe',
 //   email: 'john@example.com',
 //   salary: { amount: 120000, currency: 'USD' }

content/docs/protocol/objectql/types.mdx

@@ -628,12 +628,12 @@ const activity = await ObjectQL.findOne('activity', id, {
   expand: ['parent']  // Resolves based on _type
 });
 
-// activity.parent = { _id: '123', _type: 'account', company_name: 'Acme' }
+// activity.parent = { id: '123', _type: 'account', company_name: 'Acme' }
 ```
 
 **Database mapping:**
 - PostgreSQL: Two columns `parent_id` + `parent_type`
-- MongoDB: `{ _id: String, _type: String }`
+- MongoDB: `{ id: String, _type: String }`
 
 **Use cases:**
 - Activity feeds (related to Account OR Contact)