Add create parameter to set_pointer for auto-creating missing paths

[stefankoegl]

Summary

This PR adds a create parameter to the set_pointer() function and JsonPointer.set() method that enables automatic creation of missing intermediate members when setting values in nested structures.

Key Changes

• Added optional create=False parameter to set_pointer() function and JsonPointer.set() method
• When create=True, missing intermediate dictionary members are automatically created instead of raising a JsonPointerException
• Missing list elements can be created by referencing the end of the array using "-" or an index equal to the array length
• Refactored the set() method to extract helper methods:
  • _get_create_part(): Maps array indices at the end of a sequence to "-" for appending
  • _is_missing(): Checks if a part doesn't exist but could be added
  • _set_child(): Extracted common logic for setting values in parent containers
• Optimized creation logic to build missing structure detached and attach in one step, preserving the original document if an error occurs
• Added comprehensive test coverage for the new functionality, including edge cases and error conditions
• Updated documentation with examples of the new create parameter usage

Implementation Details

• The feature is backward compatible with create=False as the default
• When creating nested structures, the implementation builds the missing hierarchy bottom-up and attaches it in a single operation to maintain atomicity
• Proper error handling for invalid cases (e.g., creating inside strings, indices too far past array end)
• Works correctly with both inplace=True and inplace=False modes

https://claude.ai/code/session_01WGUKSut5p4NSKEm3mD5Ft3

[Wkhanex]

Ten standalone creation checks passed for escaped keys, empty keys, numeric/“-” dictionary keys, list-end append forms, inplace=False, rejected gaps/leading-zero/negative indices, and unchanged documents after invalid paths. I used Python 3.12.14 against commit ead884a.

One error-consistency edge case outside ordinary dict/list JSON is an immutable Sequence:

set_pointer({"items": ()}, "/items/0/leaf", 7, create=True)
# AttributeError: tuple has no attribute append

The new index==len branch maps 0 to “-”, and _set_child calls append without checking mutability or wrapping that failure. /items/0 behaves the same. Could immutable append targets raise JsonPointerException consistently with other invalid assignment targets? Both tuple reproductions left the document unchanged. I used fetched PR source with standalone assertions, not the full test suite.