refactor: TokenInfo readonly (800)

[AntonioCeppellini]

Description:

This PR refactor TokenInfo to make it immutable (frozen=True), removes al setter methods and replaces them with constructor initialization, and rewrites _from_proto.

• Convert TokenInfo to an immutable @dataclass(frozen=True)
• Removes all setters methods
• Rewrite _from_proto
• Add _public_key_from_oneof helper to replace the previous _copy_key_if_present
• Update all unit tests to test immutability

Related issue(s):

Fixes #800

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit tests updated and expanded)

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