refactor(sdk): Use TextMessageEventContent to send a caption

[zecakeh]

It doesn't make sense to send a formatted caption without a plain text caption so using TextMessageEventContent forces the latter to be present.

This also allows to use the helpful constructors of TextMessageEventContent.

[poljar]

Thanks, makes sense.

[codspeed-hq]

CodSpeed Performance Report

Merging #5733 will not alter performance

_{Comparing zecakeh:caption-text-message (b94cdfc) with main (681b221)}

Summary

✅ 50 untouched

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.39%. Comparing base (59b7da2) to head (b94cdfc).
⚠️ Report is 7 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5733      +/-   ##
==========================================
- Coverage   88.40%   88.39%   -0.02%     
==========================================
  Files         359      359              
  Lines       99048    99032      -16     
  Branches    99048    99032      -16     
==========================================
- Hits        87566    87540      -26     
- Misses       7344     7357      +13     
+ Partials     4138     4135       -3     

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