fix: do not panic on unresolved schema $ref during Validate

[baiyuxi930826]

Summary

Fixes #1063: Validate no longer panics when a schema $ref cannot be resolved.

SchemaFromRef can return nil for missing keys; the previous loops dereferenced the result without a nil check. Unresolved refs now add a validation error (expected schema $ref to resolve: ...) and return.

Covered sites:

• top-level ref resolution in Validate
• discriminator mapping refs
• property refs in handleMapString / handleMapAny

Test plan

• go test . -run TestValidateUnresolvedSchemaRefNoPanic
• Confirms top-level and property missing refs produce errors without panic

[codecov]

Codecov Report

❌ Patch coverage is 52.94118% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.03%. Comparing base (d6f2a37) to head (6996749).

Files with missing lines	Patch %	Lines
validate.go	52.94%	12 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1065      +/-   ##
==========================================
- Coverage   93.15%   93.03%   -0.13%     
==========================================
  Files          23       23              
  Lines        5012     5039      +27     
==========================================
+ Hits         4669     4688      +19     
- Misses        277      282       +5     
- Partials       66       69       +3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.