fix(rust): align struct fingerprint nullable flag for enum fields with Java/C++, and fix cross-language Union serialization

[ariesdevil]

Why?

When computing struct version hash (fingerprint) for cross-language compatibility, Java and C++ treat enum fields as nullable=true by default (since Java enums are reference types that can be null). However, Rust's proc-macro cannot determine at compile time whether a field type is an enum, causing fingerprint mismatch between Rust and Java/C++.

For example, a struct like:

struct Message {
    state: MyEnum,  // Java: nullable=true, Rust (before): nullable=false
    name: String,
}

Would produce different version hashes in Rust vs Java/C++, breaking cross-language schema compatibility.

What does this PR do?

This PR aligns Rust's struct fingerprint computation with Java/C++ behavior:

1. Add runtime enum detection for fingerprint computation

   • Create gen_struct_version_hash_ts() that generates code to compute fingerprint at runtime
   • For user-defined types (TypeId::UNKNOWN at macro time), call fory_static_type_id() to detect enums
   • For known primitive types, use compile-time constants to avoid unnecessary trait bounds

2. Add helper function is_enum_type_id()

   • Checks if TypeId is ENUM, NAMED_ENUM, or UNION (Union-compatible Rust enums)

3. Fix Union-compatible enum handling

   • Add xlang parameter to fory_actual_type_id() for proper TypeId selection
   • Fix enum variant meta registration to handle Union-compatible enums
   • Fix field_need_write_type_info() to handle UNION TypeId

Now Rust computes the same fingerprint as Java/C++.

Related issues

Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?
  Note: This changes the struct version hash computation for structs containing enum fields. The new hash will match Java/C++, but may differ from previous Rust versions. This is intentional for cross-language compatibility.

Benchmark

Others

It's really hard to determine this bug 😭...