fixup: add bug-test 521 and CE0126 symptom row to fix-issue skill

Address PR #521 round-3 review (ako 2026-05-07).

- **`mdl-examples/bug-tests/521-empty-java-action-typed-arg.mdl`** — new
  bug-test calling a Java action with a list parameter and a string
  parameter, both bound to MDL `empty`. Before the fix this trips
  CE0126 "Missing value for parameter X"; after the fix, both slots
  serialise as `Argument: "empty"` and `mx check` reports 0 errors.

- **`.claude/skills/fix-issue.md`** — new symptom row mapping
  CE0126 on `call java action ... ($Param = empty)` to
  `addCallJavaActionAction` and the `resolvedBasicParams` fix pattern.
  Future contributors hitting the same symptom can land on the right
  file from the table without re-deriving the diagnosis.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hjothamendix

.claude/skills/fix-issue.md

@@ -27,6 +27,7 @@ to the symptom table below, so the next similar issue costs fewer reads.
 | MDL check gives "unexpected token" on valid-looking syntax | Grammar missing rule or token | `mdl/grammar/MDLParser.g4` + `MDLLexer.g4` | Add rule/token, run `make grammar` |
 | CE7054 "parameters updated" / CE7067 "does not support body entity" after `send rest request` | `addSendRestRequestAction` emitted wrong BSON: all params as query params, BodyVariable set for JSON bodies | `mdl/executor/cmd_microflows_builder_calls.go` → `addSendRestRequestAction` | Look up operation via `fb.restServices`; route path/query params with `buildRestParameterMappings`; suppress BodyVariable for JSON/TEMPLATE/FILE via `shouldSetBodyVariable` |
 | `CREATE X` returns "already exists — use create or replace to overwrite" but OR REPLACE is not valid for that type | Error message in executor points to wrong keyword | `mdl/executor/cmd_<type>_*.go` — find the `NewAlreadyExistsMsg` call | Change hint from `or replace` to `or modify`; verify the AST stmt uses `CreateOrModify` not `CreateOrReplace` |
+| `mx check` CE0126 "Missing value for parameter X" on `call java action ... ($Param = empty)` for typed (non-entity, non-microflow) parameters | Builder emitted `BasicCodeActionParameterValue.Argument: ""` instead of the literal `"empty"` keyword | `mdl/executor/cmd_microflows_builder_calls.go` → `addCallJavaActionAction` | Capture all resolved BasicParameterType params into `resolvedBasicParams`; when bound to MDL `empty`, emit `Argument: "empty"` so Studio Pro recognises an explicit empty literal rather than treating the slot as missing |
 
 ---
 

mdl-examples/bug-tests/521-empty-java-action-typed-arg.mdl

@@ -0,0 +1,61 @@
+-- ============================================================================
+-- Bug #521: `empty` literal on a typed Java action parameter triggers CE0126
+-- ============================================================================
+--
+-- Symptom (before fix):
+--   `mx check` reports CE0126 "Missing value for parameter X" on a
+--   `call java action Module.Foo(Param = empty)` when Param is a typed
+--   (non-entity, non-microflow) parameter — most commonly a list parameter
+--   or a String parameter. The builder emitted a BasicCodeActionParameterValue
+--   with `Argument: ""` instead of the literal `"empty"` keyword Studio Pro
+--   uses to mark an explicit empty literal. Studio Pro treats the empty
+--   string as a missing slot and fires CE0126 on validation.
+--
+-- After fix:
+--   When the backend resolves the Java action and the parameter is a
+--   BasicParameterType (StringType, ListType, ParameterizedEntityType, etc.),
+--   the builder emits `Argument: "empty"` for an MDL `empty` literal so
+--   Studio Pro reads the slot as an explicitly-empty value, not a missing
+--   one. Entity-typed parameters keep using EntityCodeActionParameterValue
+--   and microflow-typed parameters keep using MicroflowParameterValue, so
+--   this fix is scoped to BasicParameterType alone.
+--
+-- Validation:
+--   `mxcli check 521-empty-java-action-typed-arg.mdl` parses the script.
+--   `mx check` against the resulting MPR reports 0 errors for both the
+--   list-typed and string-typed `empty` arguments. Roundtrip preserves
+--   the `Argument: "empty"` BSON value byte-for-byte.
+--
+-- Usage:
+--   mxcli exec mdl-examples/bug-tests/521-empty-java-action-typed-arg.mdl -p app.mpr
+--   mxcli -p app.mpr -c "describe microflow BugTest521.MF_CallWithEmpty"
+-- ============================================================================
+
+create module BugTest521;
+
+create non-persistent entity BugTest521.Item (
+  Code : string
+);
+/
+
+-- Java action with a list parameter and a string parameter — both typed
+-- (BasicParameterType) so they exercise the resolved-basic-arg path.
+create java action BugTest521.JA_ProcessBatch(Items: list of BugTest521.Item not null, Tag: string not null) returns boolean
+as $$
+return true;
+$$;
+/
+
+-- Calling with `empty` for both typed slots must produce
+-- BasicCodeActionParameterValue.Argument = "empty" so Studio Pro
+-- recognises the explicit empty literal and does not fire CE0126.
+create microflow BugTest521.MF_CallWithEmpty ()
+returns boolean
+begin
+  $Result = call java action BugTest521.JA_ProcessBatch(
+    Items = empty,
+    Tag = empty)
+    on error rollback;
+  return $Result;
+end;
+/