Add yaml.is_valid builtin

[Aayush10016]

Summary

• adds the yaml.is_valid builtin to opa-builtins-json
• validates YAML strings with the existing Jackson YAML mapper
• adds focused tests for registration, valid YAML, and invalid YAML

Closes #144.

Verification

• JAVA_TOOL_OPTIONS=-Dfile.encoding=UTF-8 .\gradlew.bat --no-daemon --console=plain :opa-builtins:opa-builtins-json:test
• JAVA_TOOL_OPTIONS=-Dfile.encoding=UTF-8 .\gradlew.bat --no-daemon --console=plain :opa-builtins:opa-builtins-json:check

Note: On this Windows environment, the first Gradle run without explicit UTF-8 failed while compiling existing Unicode box-drawing characters in MetricsPrinter.java using Windows-1252. Rerunning with UTF-8 completed successfully.

[sspaink]

@Aayush10016 did you mean to open this as a draft?

[Aayush10016]

It was initially opened as a draft while validation and DCO were being finalized. It is now marked ready for review, and the current checks are passing. Thanks for checking.

[sspaink]

Thank you for working on this!

[sspaink]

OPA returns false for non-string operands rather that raising a type error. Can you catch that here and return false instead? Adding a test to cover this would also be nice.

if (!(args[0] instanceof RegoString yamlInput)) {
  return RegoBoolean.FALSE;
}

[Aayush10016]

Thanks for the review. I pushed an update that:

• returns false from yaml.is_valid when the operand is not a RegoString
• adds a non-string input test
• updates the JUnit/JUnit Platform versions in opa-builtins-json to 6.1.2

Validation passed locally:

• :opa-builtins:opa-builtins-json:test --tests io.github.open_policy_agent.opa.ast.builtin.impls.JsonBuiltinsTest
• :opa-builtins:opa-builtins-json:test

DCO is passing now as well.

[sspaink]

Great thank you!

[sspaink]

Suggested change

	testImplementation("org.junit.jupiter:junit-jupiter:6.1.1")
	testRuntimeOnly("org.junit.platform:junit-platform-launcher:6.1.1")
	testImplementation("org.junit.jupiter:junit-jupiter:6.1.2")
	testRuntimeOnly("org.junit.platform:junit-platform-launcher:6.1.2")

the rest of the repo is on 6.1.2