DESCRIBE PAGE does not read back expression-type pluggable widget properties #1586

DESCRIBE PAGE silently omits any property whose type is expression in the widget's property schema. The property is correctly written to the .mpr by exec, but
it never appears in the DESCRIBE PAGE output of subsequent runs. This gives a false impression that the value was dropped.

Reproduction steps

1. Write and exec a page script that sets an expression-type property on a pluggable widget, for example:

mdl
entitypicker ep (
...
Placeholder: "'Record: ' + $currentObject/NId",
...
)
2. Immediately after exec, run:

./mxcli -p .mpr -c "DESCRIBE PAGE ."
3. The Placeholder property (and any other expression-type property) is absent from the output, even though exec reported no error.
4. Open the same page in Studio Pro → the Placeholder expression is visible and correct.

Expected behaviour

DESCRIBE PAGE output should include all properties that were written to the model, regardless of their schema type (expression, textTemplate, selection, etc.).

Actual behaviour

expression-type properties are silently omitted from DESCRIBE PAGE. Because the tool is the only way to inspect the model programmatically, this makes the
properties appear missing and makes it impossible to verify their value without opening Studio Pro manually.

Additional issue: misleading warning during exec

When exec writes an expression-type property, it emits a warning that the value "has not yet been persisted". This warning is factually incorrect — as
confirmed by direct inspection in Studio Pro, the value IS written to the model. The warning causes unnecessary confusion and may lead developers to conclude
that exec dropped the value.

Impact

• Automated verification of expression-type widget properties is impossible via the CLI alone.
• The misleading exec warning causes developers to believe the property was silently dropped and to seek manual workarounds in Studio Pro.
• Any DESCRIBE-based round-trip tool (diff, re-exec, review) will incorrectly flag these properties as missing.

Environment

• mxcli version: (0.26.0)
• Mendix version: 11.12.4