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

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