Rust: codegen + runtime support for vectors of unions (#5024)

[MukundaKatta]

Summary

Refs #5024. Adds Rust codegen and runtime support for vectors of unions, following the paired-vectors layout used by C++/Go/Java codegen.

What's done

• idl_parser.cpp whitelists Rust for [Union] schemas via SupportsAdvancedUnionFeatures().
• idl_gen_rust.cpp emits _type (Vector) and value vector accessors plus per-variant <field>_as_<variant>(idx) helpers.
• flatbuffers::TableVerifier::visit_union_vector runtime helper verifies discriminant and value vectors in lockstep, enforces same-length invariant (matches C++ PR Ensure the two vectors in a vector of unions are the same size in verifier #8853), and dispatches per-variant verifiers.
• 5 runtime tests covering valid round-trip, length mismatch, orphan type-vector, empty pair, and absent optional.
• RUST_VECTORS_OF_UNIONS_DESIGN.md walks the wire format, generated Rust shape, verifier contract, forward-compat handling, and aliased-variant behavior.

What's TODO

• Write-side create_vector_of_unions builder helper plus matching codegen wiring (four sites scaffolded with unimplemented!() and TODO(#5024) markers).
• Native pack/unpack for owned Vec<UnionT>.
• Adding union_vector.fbs to scripts/generate_code.py's Rust block. flatc was not available locally to regenerate .rs test files; CI will need to do that.
• JSON serialization (--rust-serialize) for [Union] fields is currently a skip_field stub.

Test plan

• cargo test --tests in tests/rust_usage_test: 322 tests pass (276 + 21 + 13 + 5 + 4 + 1 + 1 + 1). The pre-existing with_internal_capacity_preallocates_vecs lib unit test fails on master too, unrelated to this branch.

Marking draft until write-side lands.