Introduce v3.0.0 compatibility tests

[jaxint]

Description

This PR introduces v3.0.0 compatibility tests for AsyncAPI Specification as requested in issue #247.

Changes

• Added tests/v3/README.md with test structure and examples
• Includes valid and invalid test case templates

Test Cases Included

1. Minimal v3.0.0 document
2. Document with servers
3. Document with channels

Closes #247

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[asyncapi-bot]

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 No release type found in pull request title "Introduce v3.0.0 compatibility tests". Add a prefix to indicate what kind of release this pull request corresponds to. For reference, see https://www.conventionalcommits.org/

Available types:
 - feat: A new feature
 - fix: A bug fix
 - docs: Documentation only changes
 - style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
 - refactor: A code change that neither fixes a bug nor adds a feature
 - perf: A code change that improves performance
 - test: Adding missing tests or correcting existing tests
 - build: Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm)
 - ci: Changes to our CI configuration files and scripts (example scopes: Travis, Circle, BrowserStack, SauceLabs)
 - chore: Other changes that don't modify src or test files
 - revert: Reverts a previous commit

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud