fix: add OPEN to keyword rule for enum value parsing

[engalar]

Summary

• OPEN was a lexer token (used by OPEN USER TASK, OPEN WORKFLOW) but missing from the parser's keyword rule
• This caused 3-part enum values like Module.ENUM_IssueStatus.Open to fail parsing in CREATE/CHANGE statements within microflow bodies
• The fix adds OPEN to the keyword alternatives in MDLParser.g4 line 3592

Reproduction

CREATE OR REPLACE MICROFLOW MyModule.Test ()
RETURNS Nothing
BEGIN
  $Issue = CREATE MyModule.OrderIssue (
    IssueNumber = 'TEST',
    Status = MyModule.ENUM_IssueStatus.Open,
    IssueType = MyModule.ENUM_IssueType.DeliveryDelay
  );
END;

Before: line 6:47 mismatched input '.' expecting ')'
After: Syntax OK

Test plan

• mxcli check passes for enum values with Open (previously reserved keyword)
• Existing tests still pass
• OPEN USER TASK and OPEN WORKFLOW syntax unaffected

🤖 Generated with Claude Code

[ako]

Surgical, correct fix. The diagnosis is right — OPEN was added as a lexer token in #159 but never wired into the keyword rule, breaking enum values like Module.ENUM_Status.Open. This is exactly the friction I flagged in the #159 review about common-word tokens.

Concerns

Test plan boxes are unchecked. [ ] Existing tests still pass and [ ] OPEN USER TASK syntax unaffected should be verified before merge. The change is generally safe (allows OPEN as identifier in identifier-position contexts) but worth confirming.

No regression test added. A doctype-test with an enum value named Open would prevent regression — even one line in an existing test file would do it.

Same class of issue likely affects other tokens from #159. That PR added DATA, RECORDS, NOTIFY, PAUSE, UNPAUSE, ABORT, RETRY, RESTART, LOCK, UNLOCK, REASON — common words that could appear as enum values, attribute names, etc. I'd recommend either:

1. Adding all of them to the keyword rule in this PR (zero risk, broad coverage)
2. Filing a follow-up to audit and add regression tests

LGTM for the immediate fix.