fix(chat): prevent duplicate context element entries

[planger]

What it does

Prevents duplicate context variable requests in the AI chat input (filtering by variable and arguments).

How to test

Try adding files multiple times into the chat widget (by drag and drop, by auto-complete) and observe that it won't add it multiple times in the list.

Note that I kept adding it multiple times in the text, as users may want to use drag and drop to create the variable string for refering to it in their user request.

Follow-ups

None

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

None

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[colin-grant-work]

Changes look good to me - good to keep the context cleaner.