Add functional tests for per-message export (happy path + auth/ownership failures)

[Copilot]

Review feedback on #783 requested a standalone functional test script covering the per-message export flows (client-side Markdown + Word backend), including at least one happy path and a basic auth/ownership failure case.

Added: functional_tests/test_per_message_export.py

• Happy path – Word export: constructs a .docx from a valid message/conversation pair, verifies non-empty output and round-trip heading presence; skips gracefully if python-docx is absent
• Happy path – Markdown export: validates the formatted output (### Role, *timestamp*, content) matching the client-side logic in chat-message-export.js
• Auth failure: None/empty user_id → 401 "User not authenticated"
• Ownership failure – wrong user: caller requesting another user's conversation → 403 "Access denied"
• Ownership failure – missing conversation: Cosmos lookup returns nothing → 404 "Conversation not found"
• Content normalisation: covers all three input shapes (plain string, list of content-part dicts, single dict) including image_url entries rendering as [Image]
• Markdown without timestamp: confirms the *timestamp* line is suppressed when absent

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