Preserve document follow up type when merging queued gateway events

[hharry11]

Summary

Preserve non-text media semantics when merging queued gateway follow-ups.

Previously, if a busy session queued a text follow-up and then received a document follow-up, the merged pending event could remain typed as TEXT. That caused downstream document-specific handling to be skipped, so the attachment was not treated as a document on the next turn.

Root cause

merge_pending_message_event() only promoted merged events to PHOTO. For other media types, a queued TEXT event could keep its original type even after inheriting document media payloads.

Fix

When merging into an existing queued event:

• keep the existing PHOTO precedence behavior unchanged
• if the existing pending event is TEXT and the incoming event is a non-text media type, promote the merged event to the incoming message type

This keeps the fix narrow and avoids broader refactoring of merge precedence.

Testing

Added regression test:

• tests/gateway/test_session_race_guard.py::test_merge_pending_message_event_promotes_document_followups_over_text

Verified result:

• 1 passed

[teknium1]

Merged via #18215 — your commit was cherry-picked onto current main with authorship preserved in git log. Thanks for the fix!

#18215