[py] reject negative enum and union index in binary decoder

[arib06]

What is the purpose of the change

read_enum, read_union and skip_union in the Python binary decoder read the enum symbol index / union branch index from the wire and validate it with an upper-bound-only check (index >= len(...)). A negative index is not caught. Because the index is then used directly to subscript writers_schema.symbols / writers_schema.schemas, and Python lists accept negative subscripts, a crafted negative zigzag index passes validation and wraps to an element counted from the end of the list. For an enum this silently returns the wrong symbol; for a union it selects a branch that the writer never declared and decodes the following bytes with that wrong branch schema, which is a decode-time type confusion and desyncs the stream for skip_union. Positive out-of-range indices are already rejected, so this only closes the negative side. The check becomes 0 <= index < len(...) at all three sites.

Verifying this change

This change added tests and can be verified as follows:

• Added test_negative_enum_index and test_negative_union_index in TestMisc, feeding a single 0x01 byte (zigzag -1) as the index. Both assert SchemaResolutionException. Against the unpatched decoder the enum read returns symbols[-1] and the union read decodes via schemas[-1], so both tests fail before the fix.
• Full test_io suite green (152 tests).

Documentation

• Does this pull request introduce a new feature? no

[RyanSkraba]

Ooof, my apologies: it looks like these changes are all contained in #3861 submitted last week. It adds a lot of additional detailed validation around potentially invalid data, and it seems like the right PR to merge.

Please don't be discouraged, there's plenty of bugs remaining to fix 😄 For what it works, this one LGTM.

[arib06]

@RyanSkraba understood, and thankyou for informing. i will be looking forward to keep on contributing .

[RyanSkraba]

If you're a strong python developer, we'd love some help on PR reviews (including #3861)

[arib06]

Happy to help on reviews. Left comments on #3861 — the index checks there cover the same three sites this PR did, plus a lot more, so no complaint about closing this one. Flagged one perf regression in the collection path with numbers and a suggested fix, otherwise it looks good to me.