fix(regex): register provider and fix match/compile parity - #193

The BuiltinProvider entry for opa-builtins-regex was commented out in
its META-INF/services file, so ServiceLoader never discovered it and
none of the eight regex builtins were reachable for consumers.

Registering it exposed two parity gaps the fixtures had been skipping:

  - regex.match used Matcher.matches(), which requires the pattern to
    match the entire input. Go's regexp.MatchString searches instead, so
    regex.match("", "x") must be true and an unanchored pattern must
    match a substring.

  - An invalid pattern let Java's unchecked PatternSyntaxException
    escape and abort evaluation. OPA reports this as a builtin error, so
    the call is undefined by default and only aborts under
    strict-builtin-errors. All six compile sites now go through a helper
    that converts it to BuiltinError.

Adds RegexBuiltinsTest, including a ServiceLoader assertion: the module
had no unit tests, and the existing ones elsewhere call impl classes
directly, which is why the registration gap went unnoticed. Eight of the
ten new tests fail against the unfixed code.

Removes the eight regex entries from known-missing-builtins.txt (64 -> 56).

Signed-off-by: Sebastian Spaink <sebastianspaink@gmail.com>