Fix: fix Hub thread safety.

[maciejwalkowiak]

📜 Description

Fix: fix Hub thread safety.

💡 Motivation and Context

WeakHashMap is not thread safe and must be synchronized.

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

[codecov-io]

Codecov Report

Merging #1388 (6f816f9) into main (972a01b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1388   +/-   ##
=========================================
  Coverage     75.83%   75.83%           
  Complexity     1879     1879           
=========================================
  Files           190      190           
  Lines          6413     6414    +1     
  Branches        643      643           
=========================================
+ Hits           4863     4864    +1     
  Misses         1262     1262           
  Partials        288      288           

Impacted Files	Coverage Δ	Complexity Δ
sentry/src/main/java/io/sentry/Hub.java	74.35% <100.00%> (+0.07%)	76.00 <1.00> (ø)

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