feat(search): index media, stickers, polls and more message types

[stefanceriu]

Previously the search index only handled plain-text m.room.message events; everything else hit MessageTypeNotSupported and was silently dropped.

This decouples the index from ruma event types and broadens coverage with the search crate now being content-agnostic. RoomIndexOperation::Add/Edit carry a new IndexableEvent instead of OriginalSyncRoomMessageEvent and make_doc builds a document straight from that struct. The logic for extracting the text from the messages is now within the main SDK crate.

Types added:

• Image, Video, Audio and File for filename + caption
• Emote, Notice and ServerNotice for body
• Location for description
• Sticker for Sticker and alt text
• and finally Polls for question and answers

• I've documented the public API changes in the appropriate changelog files (see [Writing changelog entries][pull-request-template-changelog]).
• This PR was made with the help of AI.

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.94%. Comparing base (97380ac) to head (30c30ff).
⚠️ Report is 13 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/search_index/mod.rs	92.26%	9 Missing and 6 partials ⚠️
crates/matrix-sdk-search/src/index/mod.rs	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6710      +/-   ##
==========================================
+ Coverage   89.91%   89.94%   +0.02%     
==========================================
  Files         396      396              
  Lines      110609   110800     +191     
  Branches   110609   110800     +191     
==========================================
+ Hits        99454    99656     +202     
+ Misses       7378     7360      -18     
- Partials     3777     3784       +7     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing stefan/searchIndexMoreMessageTypes (30c30ff) with main (805f981)}