Implement rego metadata and module parsing builtins (rego.*)

Missing builtins

• rego.metadata.chain
• rego.metadata.rule
• rego.parse_module

Context

These OPA builtins are not implemented in the SDK. Most are exercised by the Rego compliance suite (opa-evaluator/src/test/resources/compliance/.../TestData) and currently raise FunctionNotFoundError; implementing them makes those cases pass.

rego.metadata.* requires exposing rule/package annotations to the evaluator.

Implementation notes

Register each builtin with an @OpaBuiltin-annotated static method (see io.github.open_policy_agent.opa.ast.builtin.impls). Core builtins are listed in BuiltinRegistry.BUILTIN_CLASSES; domain-specific ones live in an opa-builtins-* module exposed via the BuiltinProvider SPI (e.g. RegexBuiltins, CidrBuiltins, SemverBuiltins).

Reference: https://www.openpolicyagent.org/docs/policy-reference/#built-in-functions

Start by reading the builtin implementations under io.github.open_policy_agent.opa.ast.builtin.impls and the registration path in BuiltinRegistry.BUILTIN_CLASSES. Compare the SPI examples such as RegexBuiltins, CidrBuiltins, and SemverBuiltins, then inspect the compliance cases under opa-evaluator/src/test/resources/compliance/.../TestData. Done means the three rego.* builtins are registered, metadata annotations are exposed to the evaluator, and the affected compliance cases no longer raise FunctionNotFoundError.