Replace bare type assertions with errors.As in request/response validation

[SebTardif]

Problem

ValidateResponseSchema and ValidateRequestSchema use bare type assertions to cast the error returned by compiledSchema.Validate():

jk := scErrs.(*jsonschema.ValidationError)

If Validate() ever returns an error that is not exactly *jsonschema.ValidationError (e.g., a wrapped error in a future library version), this panics with a runtime type assertion failure.

The same operation in validate_schema.go:247-248 already uses the safe pattern:

var jk *jsonschema.ValidationError
if errors.As(scErrs, &jk) {

The bare assertions were introduced in PR #18 (2023-08-07) and PR #20 (2023-09-02).

Fix

Replace both bare type assertions with errors.As, following the existing pattern in validate_schema.go. When the assertion fails, the function still reports a validation error but with nil SchemaValidationErrors, consistent with other error paths in the codebase.

Testing

All existing tests pass. The fix cannot be demonstrated with a red-green test because compiledSchema is a concrete type from santhosh-tekuri/jsonschema/v6 and cannot be mocked to return a different error type. This is a defensive fix following the established codebase pattern.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.08%. Comparing base (f95f219) to head (5928f18).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #270   +/-   ##
=======================================
  Coverage   98.08%   98.08%           
=======================================
  Files          64       64           
  Lines        6987     6989    +2     
=======================================
+ Hits         6853     6855    +2     
  Misses        133      133           
  Partials        1        1           

Flag	Coverage Δ
unittests	98.08% <100.00%> (+<0.01%)	⬆️

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