feat(call): introduce StreamingHandler, HandlerKind, ResponseStream + INVALID_OPERATION_TYPE (ADR-049)

Add the foundational types for ADR-049 streaming handlers:
- StreamingHandler, ResponseStream type aliases and HandlerKind enum
  (Once | Stream) in registration.rs, with make_streaming_handler() helper
- CallError::invalid_operation_type() in wire.rs (sixth protocol code,
  retryable: false)
- HandlerRegistration.handler flipped from Handler to HandlerKind;
  HandlerRegistration::new() now takes HandlerKind
- OperationRegistryBuilder absorbs wrapping: with_local/with_leaf/
  with_leaf_provenance wrap raw Handler in HandlerKind::Once for
  Query/Mutation; new with_local_streaming/with_leaf_streaming take a
  StreamingHandler and wrap in HandlerKind::Stream for Subscription.
  Builder validates kind matches spec.op_type (mismatch = startup error)
- OperationRegistry::register() returns Result<(), String> with a clear
  mismatch message; all call sites updated to handle the Result
- invoke() matches on HandlerKind: Once -> existing path; Stream ->
  INVALID_OPERATION_TYPE error envelope (guards against silent
  truncation; invoke_streaming() added in a downstream task)
- OverlayOperationEnv::invoke_with_policy matches on HandlerKind:
  Once -> dispatch; Stream -> INVALID_OPERATION_TYPE (composition is
  request/response-only)
- Migrated every HandlerRegistration::new() construction site (~95)
  to wrap raw Handler in HandlerKind::Once(handler); the builder sites
  are handled by the builder-absorbs-wrapping change
- Updated two websocket subscription tests that relied on Subscription
  ops dispatching via invoke() to expect INVALID_OPERATION_TYPE
- Added unit tests for invoke/register validation and
  make_streaming_handler

crates/alknet-http/src/websocket/mod.rs

@@ -18,7 +18,7 @@ mod tests {
     use alknet_call::protocol::wire::{EventEnvelope, ResponseEnvelope, EVENT_RESPONDED};
     use alknet_call::registry::context::AbortPolicy;
     use alknet_call::registry::registration::{
-        make_handler, HandlerRegistration, OperationProvenance,
+        make_handler, HandlerKind, HandlerRegistration, OperationProvenance,
     };
     use alknet_call::registry::spec::{AccessControl, OperationSpec, OperationType, Visibility};
     use alknet_core::auth::{Identity, IdentityProvider};
@@ -77,14 +77,18 @@ mod tests {
 
     fn echo_registry() -> Arc<alknet_call::registry::registration::OperationRegistry> {
         let mut registry = alknet_call::registry::registration::OperationRegistry::new();
-        registry.register(HandlerRegistration::new(
-            external_spec("echo/run"),
-            make_handler(|input, ctx| async move { ResponseEnvelope::ok(ctx.request_id, input) }),
-            OperationProvenance::Local,
-            None,
-            None,
-            Capabilities::new(),
-        ));
+        registry
+            .register(HandlerRegistration::new(
+                external_spec("echo/run"),
+                HandlerKind::Once(make_handler(|input, ctx| async move {
+                    ResponseEnvelope::ok(ctx.request_id, input)
+                })),
+                OperationProvenance::Local,
+                None,
+                None,
+                Capabilities::new(),
+            ))
+            .unwrap();
         Arc::new(registry)
     }
 
@@ -174,7 +178,9 @@ mod tests {
         assert!(!env.contains("worker/exec"));
         conn.register_imported(HandlerRegistration::new(
             external_spec("worker/exec"),
-            make_handler(|input, ctx| async move { ResponseEnvelope::ok(ctx.request_id, input) }),
+            HandlerKind::Once(make_handler(|input, ctx| async move {
+                ResponseEnvelope::ok(ctx.request_id, input)
+            })),
             OperationProvenance::FromCall,
             None,
             None,