feat: add reusable session cloning endpoint

[anahaaaa]

Description

Adds a reusable session cloning endpoint to duplicate an existing chat session.

Closes #295

Changes

• Added POST /api/sessions/{session_id}/clone

• Creates a new session with:

  • A new unique session ID
  • The original session's model
  • The original session title with " (Copy)" appended

• Copies all chat messages from the source session into the cloned session

• Returns the newly created session

• Added tests for:

  • Successful session cloning
  • Handling non-existent sessions (404)

Example

Source session:

• Title: Original Chat

• Messages:

  • User: Hello
  • Assistant: Hi there

Cloned session:

• Title: Original Chat (Copy)

• Messages:

  • User: Hello
  • Assistant: Hi there

Verification

• Added automated test coverage for clone functionality.
• All existing tests pass successfully (41 passed).

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