fix(protobuf): reject cyclic collections when decoding Struct/Value

[Amayyas]

Decoding a cyclic Dart collection recursed until the stack overflowed:

final map = <String, Object?>{};
map['x'] = map;
Value.fromJson(map); // StackOverflowError

Value, Struct and ListValue decode into each other, so the decoding functions now thread the set of containers currently being decoded through the recursion, and throw a FormatException when one is re-entered.

Notes

• Lists have the same problem, as do cycles that span both (a map holding a list that holds the map). Those are covered too, since the fix sits on the shared path.
• FormatException was chosen for consistency with the existing behaviour for other undecodable input — Value.fromJson(DateTime.now()) already throws it. Happy to switch to ArgumentError if you'd rather treat it as a caller-side mistake.
• Repeated-but-acyclic input still decodes. Entries leave the set once decoded, so only ancestors are rejected — e.g. the same map referenced by two keys is fine. Covered by tests.
• Only decoding is affected. Struct.fields is final, so a cyclic object graph cannot be constructed in the first place and encoding cannot recurse.

dart format, dart analyze, dart test (180 passing) and the protojson conformance suite (211 passing) are all clean.

Fixes #285

[gemini-code-assist]

Code Review

This pull request adds unit tests to verify the behavior of JSON decoding for cyclic and repeated acyclic structures in Struct, ListValue, and Value classes. Specifically, it introduces tests to ensure that cyclic maps and lists are rejected with a FormatException, while repeated but acyclic references are successfully decoded. There are no review comments, and I have no additional feedback to provide.