fix(core): implement ChatPromptTemplate.save() method (#32637)

[cwarre33]

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

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[codspeed-hq]

CodSpeed Performance Report

Merging #33631 will improve performances by 11.07%

_{Comparing cwarre33:fix-chat-prompt-template-save-32637 (e00f3ff) with master (5acd34a)¹}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

⚡ 1 improvement
✅ 12 untouched
⏩ 21 skipped²

Benchmarks breakdown

	Mode	Benchmark	BASE	HEAD	Change
⚡	WallTime	test_import_time[BaseChatModel]	562.3 ms	506.3 ms	+11.07%

Footnotes

1. No successful run was found on master (f8adbbc) during the generation of this report, so 5acd34a was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 21 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩