[API View] Validate and Enforce No Duplicate Line IDs

## Summary
While reviewing a code file in API View, I encountered a bug caused by duplicate element IDs in the following revision:
🔗 https://spa.apiview.dev/review/8129e6c762924e02836a2e79b2b480a6?activeApiRevisionId=55c25374ba80457c94d26ddb6d83600e
This issue originated from the Python parser, and it caused the "Generate Copilot Review" flow to fail.

## Problem
- The error is not clearly communicated to users.
- The failure occurs late in the flow, making it harder to debug.
- There is no early validation to catch duplicate line IDs.

## Proposal
Introduce validation when a code file is created to:
- Detect duplicate line IDs early.
- Display a clear error message that:
- Identifies the specific duplicate line ID.
- Helps users understand that the parser needs to be fixed.

Start at the API View code-file creation path and reproduce the failure using the linked revision. Trace how line IDs from the Python parser are handled; done means duplicate IDs are rejected early with an error naming the duplicated ID and directing users to fix the parser.

Written by the indexing model from the issue text.