Add host value callbacks to bridges

Author: 2kai2kai2

baml_language/crates/baml_builtins2/baml_std/baml/ns_errors/errors.baml

@@ -56,6 +56,7 @@ class HostPanic {
 }
 
 class HostCallable {
+  message string
   class_name string
   language string
   traceback string?

baml_language/crates/baml_tests/projects/compiles/host_callable_call/main.baml

@@ -0,0 +1,11 @@
+// snapshot the bytecode shape for a host-callable call site.
+//
+// A `Ty::Function` parameter is bound at the FFI boundary to an
+// `Object::HostClosure`; calling it via `f(x)` lowers to a plain
+// `CallIndirect` instruction at compile time. The HostClosure-specific
+// dispatch (yielding `VmExecState::SysOp { operation:
+// SysOp::BamlHostCallHostValue, args }`) is entirely a runtime concern
+// — the compiler emits no synthesized wrapper for this call.
+function call_with_callback(callback: (int) -> string, x: int) -> string {
+    callback(x)
+}