fix(idl): preserve legacy type generics during IDL conversion

[akash-osec]

Fixes finding AI # 15

Legacy IDL conversion dropped type-parameter declarations while keeping generic field uses, so declare_program! emitted undeclared T/N. Declarations are now mapped to v2 IdlTypeDefGeneric::Type and covered by a converter regression test.

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Additional details and impacted files

@@              Coverage Diff               @@
##             anchor-next    #4922   +/-   ##
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  Coverage               ?   64.64%           
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  Files                  ?      127           
  Lines                  ?    16612           
  Branches               ?        0           
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  Hits                   ?    10738           
  Misses                 ?     5874           
  Partials               ?        0           

Flag	Coverage Δ
v2	64.64% <100.00%> (?)

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