fix(lang-v2): enforce PodVec MAX fits u16 length prefix

[akash-osec]

Fixes finding AI # 43:

PodVec stored len as u16 but accepted any MAX, so oversized vectors could be declared and advertise capacity until a mutation forced the assert. Now, it force the bound via CAPACITY/Default/reads and reject oversized PodVec fields in #[account].

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Codecov Report

❌ Patch coverage is 47.22222% with 19 lines in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
lang-v2/derive/src/lib.rs	34.48%	19 Missing ⚠️
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Additional details and impacted files

@@              Coverage Diff               @@
##             anchor-next    #4907   +/-   ##
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  Coverage               ?   64.99%           
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  Files                  ?      127           
  Lines                  ?    15892           
  Branches               ?        0           
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  Hits                   ?    10329           
  Misses                 ?     5563           
  Partials               ?        0           

Flag	Coverage Δ
v2	64.99% <47.22%> (?)

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