fix(core): JSON Schema reference resolution for list indices (#32088)

Fixes #32042

## Summary
Fixes a critical bug in JSON Schema reference resolution that prevented
correctly dereferencing numeric components in JSON pointer paths,
specifically for list indices in `anyOf`, `oneOf`, and `allOf` arrays.

## Changes
- Fixed `_retrieve_ref` function in
`libs/core/langchain_core/utils/json_schema.py` to properly handle
numeric components
- Added comprehensive test function `test_dereference_refs_list_index()`
in `libs/core/tests/unit_tests/utils/test_json_schema.py`
- Resolved line length formatting issues
- Improved type checking and index validation for list and dictionary
references

## Key Improvements
- Correctly handles list index references in JSON pointer paths
- Maintains backward compatibility with existing dictionary numeric key
functionality
- Adds robust error handling for out-of-bounds and invalid indices
- Passes all test cases covering various reference scenarios

## Test Coverage
- Verified fix for `#/properties/payload/anyOf/1/properties/startDate`
reference
- Tested edge cases including out-of-bounds and negative indices
- Ensured no regression in existing reference resolution functionality

Resolves the reported issue with JSON Schema reference dereferencing for
list indices.

---------

Co-authored-by: open-swe-dev[bot] <open-swe-dev@users.noreply.github.com>
Co-authored-by: Mason Daugherty <github@mdrxy.com>
Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/langchain/langchain/utils/json_schema.py

@@ -1,13 +1,11 @@
 from langchain_core.utils.json_schema import (
     _dereference_refs_helper,
-    _infer_skip_keys,
     _retrieve_ref,
     dereference_refs,
 )
 
 __all__ = [
     "_dereference_refs_helper",
-    "_infer_skip_keys",
     "_retrieve_ref",
     "dereference_refs",
 ]