fix(validator): reject type "null" in OAS 3.0 documents

[erraggy]

Summary

• OAS 3.0.x only allows type to be one of array, boolean, integer, number, object, string. The "null" type was introduced in OAS 3.1+ (JSON Schema 2020-12). Previously the validator silently accepted type: "null" on 3.0 documents.
• Plumb the parsed OAS version onto the Validator via a new oasVersion field (set in ValidateParsed) and add a "null" case to validateSchemaTypeConstraints that emits an error when the document is OAS 3.0.x.
• OAS 3.1+ continues to accept both scalar type: "null" and the type-array form (e.g. ["string", "null"]) — this PR only tightens the 3.0 path.

Fixes #362.

Error message

✗ null is not a valid type for OpenAPI 3.0; valid types are: array, boolean, integer, number, object, string. Use "nullable: true" instead.

Scope note

The issue mentions a related pattern — specs that carry both nullable: true (3.0 idiom) and anyOf: [{type: "null"}] (3.1 idiom). That cross-idiom check is intentionally deferred to keep this PR tight and focused on the primary reported bug.

Test plan

• New TestValidate_NullType_OAS30_Rejected covers all 3.0.x minor versions (3.0.0 through 3.0.4)
• TestValidate_NullType_OAS31_Allowed regression guard — 3.1.0 still accepts type: "null"
• TestValidate_StringType_OAS30_Allowed regression guard — valid 3.0 types still pass
• make check passes locally (lint 0 issues, 8568 tests, validator coverage 89.4%)
• CI green

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: e0682a14-0850-4ba8-9dfe-6ca8db338a44

📥 Commits

Reviewing files that changed from the base of the PR and between 91814fe and 70e824b.

📒 Files selected for processing (3)

• validator/schema.go
• validator/schema_test.go
• validator/validator.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/validator-null-type-oas30

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.76%. Comparing base (91814fe) to head (70e824b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #365   +/-   ##
=======================================
  Coverage   84.75%   84.76%           
=======================================
  Files         194      194           
  Lines       27458    27469   +11     
=======================================
+ Hits        23272    23283   +11     
  Misses       2842     2842           
  Partials     1344     1344           

Files with missing lines	Coverage Δ
validator/schema.go	65.38% <100.00%> (+2.37%)	⬆️
validator/validator.go	92.80% <100.00%> (+0.05%)	⬆️

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