[FFI] Add ref-qualified strict ObjectRef casts

[tqchen]

This PR refactors strict ObjectRef casting to use the Any TypeTraits strict-check path and adds strict throwing APIs.

Main benefit:

• Any and ObjectRef now have consistent strict as / as_or_throw APIs: both rely on the same TypeTraits strict-check semantics, both return optional values for probe-style as<T>(), and both provide throwing as_or_throw<T>() variants for required casts.
• ObjectRef casts can now support richer compatibility checks through TypeTraits<ObjectRefType>::CheckAnyStrict, instead of being limited to the target ref's canonical ContainerType instance check.
• ObjectRefType::ContainerType now has an explicit _type_container_is_exact invariant. Ordinary refs inherit true from ObjectRef, while richer parameterized refs such as Array<T>, Map<K,V>, Tuple<...>, and object-backed Variant<...> opt out because their accepted values are determined by TypeTraits, not only by the backing container. GetRef is guarded to only work for exact-container refs.

API behavior:

• ObjectRef::as<ObjectRefType>() const& returns std::optional<ObjectRefType>. For a non-null source, it succeeds only when TypeTraits<ObjectRefType>::CheckAnyStrict accepts the object through a compact temporary TVMFFIAny view. On success, it returns a ref that shares the original object pointer. On type mismatch, it returns std::nullopt.
• ObjectRef::as<ObjectRefType>() && has the same strict-check and std::nullopt behavior, but moves the object pointer into the returned ref on success and clears the source ref.
• For null ObjectRef sources, ObjectRef::as<ObjectRefType>() returns a successful null ref when the target ref type is nullable, and returns std::nullopt for non-nullable target refs. The null path is explicit and is not treated as a globally cold failure path.
• ObjectRef::as_or_throw<ObjectRefType>() const& and && are the throwing forms of the same strict ObjectRef cast. They return ObjectRefType on success, preserve/move the object pointer according to the receiver qualifier, return a null ref for nullable null targets, and throw TypeError for non-nullable null or type-mismatch cases.
• Any::as_or_throw<T>() const& and && are strict reinterpretation helpers for Any. They call the corresponding strict as<T>() path, return T on success, and throw TypeError on mismatch. They do not run fallback conversions; use cast<T>() when conversion is intended.
• Optional-returning as APIs intentionally do not use success/failure prediction annotations because std::nullopt can be a normal probe result. Throwing APIs mark only the final failure/throw path as cold, and ObjectRef::as_or_throw marks the strict-check success path as likely because mismatch throws.

Implementation notes:

• ObjectRef::as<T>() no longer relies on T::ContainerType as the complete runtime compatibility test. It piggy-backs on Any TypeTraits strict checks, so ref types with richer strict compatibility rules work consistently with Any.
• The temporary TVMFFIAny setup is deliberately kept inline in the non-null ObjectRef paths. This preserves explicit null behavior and lets as_or_throw call TypeTraits<T>::GetMismatchTypeInfo for richer diagnostics instead of reducing to as<T>() and losing the synthesized Any view.
• The compact temporary Any view is expected to optimize away on hot paths; GCC/Clang assembly probes confirmed direct object-header loads/type-index comparisons for the checked ObjectRef paths.

Changes:

• Add ref-qualified ObjectRef::as<T>() and ObjectRef::as_or_throw<T>() overloads for const and rvalue receivers.
• Add Any::as_or_throw<T>() const/rvalue helpers.
• Keep ObjectRef null handling explicit while using temporary TVMFFIAny views for rich TypeTraits checks and mismatch messages.
• Add _type_container_is_exact and guard GetRef so it is only used for refs whose ContainerType is an exact acceptance predicate.
• Add focused tests for const and move variants of as<T>() and as_or_throw<T>(), plus compile-time coverage for exact/non-exact container-ref flags.
• Update TVM_FFI_UNSAFE_ASSUME lowering to be more robust for GCC.

[gemini-code-assist]

Code Review

This pull request introduces as_or_throw() methods for both Any and ObjectRef classes to strictly reinterpret types or throw a TypeError upon failure. It also updates ObjectRef::as() to support rvalue and lvalue overloads with strict type checking, and refines the TVM_FFI_UNSAFE_ASSUME macro for GCC to use __builtin_unreachable(). The reviewer suggests simplifying the ObjectRef::as_or_throw() implementations by delegating directly to as() to eliminate duplicated low-level logic and improve maintainability.

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