fix: add ok check for CustomEventMapKey in NewFromJsonBytesContext

[afrittoli]

Summary

• The custom event path in NewFromJsonBytesContext did a map lookup on CustomEventMapKey without checking the ok flag
• If the key was absent, receiver would be the nil zero value for CDEventType
• A subsequent reflect.TypeOf(receiver).Elem() (or json.Unmarshal) would then panic
• Added the same guard used for the non-custom path: return a descriptive error instead

Test plan

• Existing tests pass (go test -tags testonly ./pkg/api/)
• No panic when a cdeventsMap without CustomEventMapKey receives a custom event

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.12%. Comparing base (3cf5657) to head (59e6ab2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #128   +/-   ##
=======================================
  Coverage   85.12%   85.12%           
=======================================
  Files         149      149           
  Lines       13095    13097    +2     
=======================================
+ Hits        11147    11149    +2     
  Misses       1631     1631           
  Partials      317      317           

Files with missing lines	Coverage Δ
pkg/api/bindings.go	83.17% <100.00%> (+0.32%)	⬆️

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