fix(langchain): Handle Non-String Dictionary Keys

[ialisaleh]

Closes #2507

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Note

Stringifies non-string dict keys and normalizes containers before JSON serialization, updating _json_dumps/safe_json_dumps and adding focused tests.

• Serialization:
  • Add _stringify_dict_keys to recursively convert dict keys to strings and normalize containers (sets→lists, tuples/lists preserved).
  • Wrap original safe_json_dumps with a new safe_json_dumps that normalizes objects first.
  • Update _json_dumps to normalize input, use custom encoder, and catch RecursionError fallback to safe_json_dumps.
  • Adjust imports to alias original safe_json_dumps as _original_safe_json_dumps.
• Tests:
  • Add test_stringify_dict_keys.py with cases covering Enums, sets/frozensets, tuples, nested structures, and primitives.

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