fix(core): implement ChatPromptTemplate.save() method (#32637)
Remove NotImplementedError override in ChatPromptTemplate.save() to allow the
parent BasePromptTemplate implementation to handle serialization. ChatPromptTemplate
already has the required _prompt_type property ('chat') for proper serialization.
This enables users to save and load ChatPromptTemplate instances to/from YAML
and JSON files, matching the functionality of PromptTemplate and
FewShotPromptTemplate.
Changes:
- Remove NotImplementedError override from ChatPromptTemplate.save()
- Add comprehensive test for save/load roundtrip with both YAML and JSON formats
Fixes #32637