Fix NPE in PAGImageView.flush() by moving bitmapCache.put() inside the bitmapLock synchronized block.

[leiyue123]

修复 PAGImageView 在开启 cacheAllFramesInMemory 时的偶发 NPE。

问题：flush() 中 bitmapCache.put(frame, renderBitmap) 在 synchronized(bitmapLock) 块外执行，而 renderBitmap 是 volatile 字段，可能被 releaseBitmap() 并发置为 null，导致 ConcurrentHashMap.put 因 value 为 null 抛出 NullPointerException。原有的 renderBitmap != null 预检查仍存在 TOCTOU 窗口，无法防护。

修复：将 bitmapCache.put 调用移入 synchronized(bitmapLock) 块内，与 releaseBitmap() 互斥；同时改用局部变量 flushBitmap 作为写入值，避免读取可被其他线程置空的共享字段。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.56%. Comparing base (2973416) to head (a154efc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3429      +/-   ##
==========================================
+ Coverage   81.55%   81.56%   +0.01%     
==========================================
  Files         558      558              
  Lines       51407    51407              
  Branches    13875    13987     +112     
==========================================
+ Hits        41924    41931       +7     
+ Misses       6603     6600       -3     
+ Partials     2880     2876       -4     

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