Skip serialized variants during type trace

[avvlas]

Summary

The tracer now records enum variants whose serialized VariantFormat is already complete, and deserialize_enum skips re-exploring those variants during type tracing. That avoids overwriting a complete serialized shape with a less precise deserialization-derived shape, while still allowing partially traced variants to be revisited and completed.

Motivation:

This change fixes the issue that can be reproduced with test: test_trace_type_skips_fully_serialized_bytes_variant

In mixed tracing flows, an enum variant may already be fully learned from trace_value before trace_type runs. For example, if tracing first sees Message::Send(Exact16([7; 16])), serialization can already determine that the variant payload is Bytes. During a later trace_type::<Message>(), re-exploring Send through deserialization is unnecessary and can be actively harmful, because the deserializer only has synthetic placeholder input and may fail or infer a weaker shape.

Test Plan

Tests added:

• test_trace_type_revisits_partially_serialized_enum_variants
• test_trace_type_skips_fully_serialized_bytes_variant

[avvlas]

Hey, @ma2bd!
Please take a look when you have time :)

[ma2bd]

@avvlas This looks good. Thanks for the PR!