fix(pdf): add 'pdfjs' element and restore native iframe PDF viewer

[Allaoua9]

This PR addresses the following issue : #2896

[cubic-dev-ai]

2 issues found across 8 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="frontend/src/components/chat/Messages/Message/Content/InlinedElements/InlinedPDFjsList.tsx">

<violation number="1" location="frontend/src/components/chat/Messages/Message/Content/InlinedElements/InlinedPDFjsList.tsx:13">
P2: Using the array index as the React key makes PDF items unstable across insertions/reorders and can reuse the wrong `PDFjsElement`/`PDFjsViewer` state.</violation>
</file>

<file name="backend/chainlit/element.py">

<violation number="1" location="backend/chainlit/element.py:37">
P2: New `pdfjs` elements are serialized with type/page metadata, but `from_dict()` does not recognize `pdfjs` and falls back to `File`, breaking deserialization round-trips for the new viewer type.</violation>
</file>

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