Fix MessageSet recursion depth propagation in Python decoder

[steadytao]

Summary

Preserve recursion depth accounting when Python MessageSet decoders recurse into unknown groups.

Problem

MessageSetItemDecoder and UnknownMessageSetItemDecoder called _DecodeUnknownField(...) without forwarding current_depth.

That reset recursion accounting in the unknown-group path and allowed MessageSet parsing to bypass the intended nesting limit in this specific case.

Fix

• forward current_depth from MessageSetItemDecoder
• forward current_depth from UnknownMessageSetItemDecoder
• add regression coverage for both paths

Testing

• added regression tests for MessageSet depth propagation
• verified the bypass path now raises DecodeError when recursion depth is already exhausted
• verified the unknown MessageSet item path also preserves recursion depth

Fixes #26437

[steadytao]

The fork safety gate appears to be blocking CI before the actual test matrix runs. Could a protobuf maintainer please add the safe for tests label once reviewed so the forked PR checks can proceed?

[esrauchg]

Thanks for the proposal!

[steadytao]

Removed the implicit =0 from UnknownMessageSetItemDecoder and updated the fresh-entry call sites to pass 0 explicitly instead of silently resetting depth. I also fixed the failing decoder test which was reaching into a runtime-specific private field.

[esrauchg]

Thanks!