Mark Source Products fields as optional

[ZeldaZach]

Fixes #662

etched, foil, and nonfoil were documented as required on the Source Products model, but MTGJSON omits any finish a card was not printed in.

Verified against live data — counting which sourceProducts keys are present per card:

Set	Key combination	Cards
10E	foil, nonfoil	258
10E	nonfoil only	125
10E	foil only	125
MH2	foil, nonfoil	377
MH2	etched, foil, nonfoil	113
MH2	foil only	1

All three fields turn up missing in practice, so each gets the optional badge. That badge is what util/generate-types.js reads, so the generated model becomes:

export type SourceProducts = {
  etched?: string[];
  foil?: string[];
  nonfoil?: string[];
};

This matches the sibling SkuIds model, which already declares its finish fields optional.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Xba7S7uYsrKAVdniqNcfhS

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d7c4aef) to head (07b3b64).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #732   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines           69        69           
  Branches        27        27           
=========================================
  Hits            69        69           

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