Fix edge case assigning new numeric types to Var/Param with units

[jsiirola]

Fixes #3100 .

Summary/Motivation:

#3100 highlighted a gap in the automatic type registration logic (which @andrewlee94 noted also led to IDAES test failures). This PR closes that hole and ensures that type registrations that occur through Param or Var set_value() are handled gracefully.

Changes proposed in this PR:

• Recheck the native_numeric_types after calling value()
• Add tests of the observed error

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