Avoid mutating typed dictionaries during restore

[jbirddog]

Summary

This change fixes a side effect in DictionaryConverter.restore() when restoring typed dictionaries.

Previously, if the input dict contained a typename, restore() removed that key from the original input via
pop(). That meant callers could get their source data mutated during deserialization.

This update changes restore() to:

• read typename without mutating the original input
• restore from a shallow copy of the typed dictionary
• keep the existing recursive restore behavior for nested values
• add a regression test covering the non-mutation behavior

It also includes a small cleanup in restore() for JSON primitive values and common container types.

Why

Deserializer helpers should not mutate caller-owned input data. That behavior is surprising, can make debugging
harder, and can break reuse of the same serialized structure across multiple restore calls or assertions.

Test Plan

• Ran:
  • uv run python -m unittest -v tests.SpiffWorkflow.bpmn.serializer.DictionaryConverterTest

[essweine]

I disagree that removing the typename is a big deal, but again, whatever. At least this is a neutral change rather that the other crap I've felt forced to merge.

[burnettk]

i've seen these _UNDERSCORE_CONSTANTS elsewhere from agents. is there some method to this madness? it's being used in the same file?