Fix render error from duplicated form

[Lainow]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !46925
• Here is a brief description of what this PR does

Fix rendering error unknown named parameters for duplicated form

Screenshots (if appropriate):

[Mary-Clb]

Regression but minor : guessing from the shape of the value whether it is "already encoded" misclassifies text defaults that happen to be valid JSON (e.g. [1,2] or {} in a text/textarea field are stored raw and rendered as Array). Unlikely in practice, but handling the decoding in the clone path only would avoid the guesswork and also fix the scalar case below.

[Mary-Clb]

Scalar defaults are still double-encoded on clone.
To reproduce : add a Fields question on a text field with the default hello, then duplicate the form. The copy shows "hello" with quotes and each further duplication adds another level of escaping (" \ "hello \ "", ...). The same applies to textarea, number, url and date fields.

Also, should existing forms already corrupted by previous clones be repaired (migration)? Tests covering the clone of scalar and dropdown defaults would help too.