Implement units.parse and units.parse_bytes builtins

[glasses-and-hat]

Summary

Implements the units.parse and units.parse_bytes OPA builtins (closes #133), which were previously unimplemented and raised FunctionNotFoundError for every call.

• Both parse strings like "10K", "1.5Gi", "100m" into a number, supporting the standard SI decimal (k/K, M, G, T, P, E) and binary (Ki, Mi, Gi, Ti, Pi, Ei) suffixes, plus the m (milli) suffix for units.parse.
• Follows the same string-scanning algorithm as the reference Go implementation (extractNumAndUnit in topdown/parse_bytes.go / topdown/parse_units.go), including scientific-notation handling and the exponent too large guard (max 6 exponent digits).
• Uses BigDecimal for exact arithmetic rather than double, to avoid floating-point precision loss on suffix multiplication (mirrors the reference implementation's use of big.Rat/big.Float):
  • units.parse_bytes truncates the product towards zero (matches big.Float.Int()).
  • units.parse returns an exact integer when the product has no fractional part, otherwise rounds to 10 decimal places (matches big.Rat.FloatString(10)).
• Error messages (no amount provided, could not parse amount to a number, spaces not allowed in resource strings, exponent too large, unit X not recognized) match the reference implementation's wording.
• Registered as core builtins in BuiltinRegistry.BUILTIN_CLASSES (no external dependencies needed, consistent with ArithmeticBuiltins/HexBuiltins).

Test plan

• All 66 pre-existing compliance test cases under opa-evaluator/src/test/resources/compliance/.../TestData/v1/units/ (covering both happy-path and error cases, including the precision-regression cases in test-units-precision.json and test-issue-4856.json) now pass — previously they were silently skipped via the compliance suite's "missing builtin" allowance.
• Confirmed units.parse/units.parse_bytes no longer appear in the compliance suite's "Missing Builtin Report".
• Full opa-evaluator test suite passes (./gradlew :opa-evaluator:test), no regressions.
• ./gradlew checkstyleMain checkstyleTest pmdMain pmdTest clean for the new file.
• ./gradlew compileJava succeeds across all modules.

[sspaink]

@glasses-and-hat can you sign you commit please for the DCO to pass? https://github.com/open-policy-agent/java-opa-sdk/pull/152/checks?check_run_id=86619295459

[glasses-and-hat]

@glasses-and-hat can you sign you commit please for the DCO to pass? https://github.com/open-policy-agent/java-opa-sdk/pull/152/checks?check_run_id=86619295459

@sspaink Done, amended the commit with a DCO sign-off and force-pushed. Should be green now, thanks for flagging!

[sspaink]

This is what's failing CI (cannot find symbol: class UnitsBuiltins). This branch still has the impls.* wildcard import, but #171 replaced it on main with explicit per-class imports — so the merge keeps main's import list (no UnitsBuiltins) alongside this new entry, and nothing imports the class. Needs a rebase on main plus the explicit import:

import io.github.open_policy_agent.opa.ast.builtin.impls.UnitsBuiltins;