feat(core): add get session graphql api (#12237)

Close [AI-116](https://linear.app/affine-design/issue/AI-116)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Added the ability to retrieve detailed information for a specific Copilot session by its ID, including model metadata and optional models, via the user interface and API.
  - Session data now includes additional fields such as the model used and a list of optional models.
  - Enhanced GraphQL queries and UI components to support fetching and displaying these new session details.

- **Improvements**
  - Session lists now provide richer information, including model details, for each session.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/schema.gql

@@ -133,6 +133,9 @@ type Copilot {
   """Get the quota of the user in the workspace"""
   quota: CopilotQuota!
 
+  """Get the session by id"""
+  session(sessionId: String!): CopilotSessionType!
+
   """Get the session id list in the workspace"""
   sessionIds(docId: String, options: QueryChatSessionsInput): [String!]! @deprecated(reason: "Use `sessions` instead")
 
@@ -318,6 +321,8 @@ type CopilotQuota {
 
 type CopilotSessionType {
   id: ID!
+  model: String!
+  optionalModels: [String!]!
   parentSessionId: ID
   promptName: String!
 }