fix(lang-v2): preserve instruction prefix coercion

[0x4ka5h]

Finding
#[instruction(...)] only accepted no arguments or the full handler tuple, so documented prefix-only argument usage failed to compile in lang-v2.

Fix
This PR restores prefix tuple coercion for partial #[instruction(...)] usage without requiring the complete handler signature. Added regression tests for prefix and full argument coercions.

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Additional details and impacted files

@@              Coverage Diff               @@
##             anchor-next    #4816   +/-   ##
==============================================
  Coverage               ?   63.60%           
==============================================
  Files                  ?      126           
  Lines                  ?    13663           
  Branches               ?        0           
==============================================
  Hits                   ?     8691           
  Misses                 ?     4972           
  Partials               ?        0           

Flag	Coverage Δ
v2	63.60% <ø> (?)

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