rx-html (fix): Properly call onMount hook when internal Rx is updated

[xerial]

• Reproduce and fixes rx-html: Failed to call onMount in nested RxElements #3118
• rx-html: Call onMount when Rx is updated

[codecov]

Codecov Report

Merging #3119 (c919a28) into main (2d9f38b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3119   +/-   ##
=======================================
  Coverage   82.84%   82.85%           
=======================================
  Files         350      350           
  Lines       14723    14723           
  Branches     2447     2447           
=======================================
+ Hits        12198    12199    +1     
+ Misses       2525     2524    -1     

Files Changed	Coverage Δ
.../main/scala/wvlet/airframe/rx/html/RxElement.scala	82.35% <100.00%> (ø)

... and 1 file with indirect coverage changes

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