fix: validate assignment target in JsonPointer.set()

[abhi-0203]

Summary

JsonPointer.set() raised raw TypeError or IndexError instead of JsonPointerException when the assignment target was invalid — strings or out-of-range list indices. This violates RFC 6901 and causes jsonpatch.apply() to crash on untrusted patch documents.

Fix

Added validation in set():

1. String guard — str is a Sequence so it passed the part == '-' check but raises TypeError on item assignment. Added explicit isinstance(parent, str) check.
2. Try/except wrapper — wrapped parent[part] = value in try/except (TypeError, IndexError) to catch any remaining invalid assignment targets and re-raise as JsonPointerException.

Reproduction (before fix)

JsonPointer("/0").set("abc", {"v": 1}, inplace=False)
# TypeError: 'str' object does not support item assignment

JsonPointer("/0").set([], 42, inplace=False)
# IndexError: list assignment index out of range

After fix

JsonPointer("/0").set("abc", {"v": 1}, inplace=False)
# JsonPointerException: Cannot set value in a string

JsonPointer("/0").set([], 42, inplace=False)
# JsonPointerException: Invalid assignment target: list assignment index out of range

All 23 existing tests pass.

[Copilot]

Copilot review overview

🟡 Changes recommended

The /- string case still leaks AttributeError, and the new behavior lacks regression tests.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Check string targets before handling the append token · New
• Add regression tests for converted JsonPointer exceptions · New

What changed in this PR

Improves JsonPointer.set() error normalization for invalid assignment targets.

Changes:

• Rejects string assignment targets.
• Converts assignment TypeError/IndexError into JsonPointerException.

File	Description
jsonpointer.py	Adds assignment-target validation and exception conversion.

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[angela-tarantula]

This is a nice catch. A style improvement would be to use raise ... from e (including more broadly in jsonpointer.py as a whole) as an observability difference between During handling of the above exception, another exception occurred: and The above exception was the direct cause of the following exception:.

[angela-tarantula]

Indentation, tests will fail.

[Copilot]

Copilot review overview

🟡 Changes recommended

Incorrect indentation prevents the module from importing, and the new behavior lacks regression tests.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Fix dedentation causing IndentationError in JsonPointer.set() · New
• Add regression tests for converted JsonPointer exceptions

Resolved since last review (1)

• Check string targets before handling the append token

[stefankoegl]

thanks!