Add integration tests against popular public OpenAPI specs (Petstore, Stripe, GitHub)

[halotukozak]

Description

IntegrationTest.kt currently tests against two internal specs (platform-api.json, analytics-api.json). Testing against well-known public API specs would increase confidence that the generator handles real-world patterns correctly. Candidates: Petstore (canonical), Stripe (complex models), GitHub (large spec with many edge cases).

Acceptance criteria

• Integration tests include at least 2 popular public OpenAPI specs
• Tests verify successful parsing (no errors) and code generation (output is non-empty)
• Known limitations are documented as warnings, not silent failures
• Spec files are vendored in test resources or fetched at build time
• Runs as part of ./gradlew test