♻️ Adopt node-graph's GraphTaskHandle for @task.graph decorators

[elinscott]

Problem

node-graph #150 made build graph-only by handle type: it removed the method (and its runtime task_type != "GRAPH" guard) from BaseHandle and moved it to a new GraphTaskHandle. node-graph's own decorator now dispatches on the spec type:

handle = GraphTaskHandle(spec) if is_graph_task else TaskHandle(spec)

aiida-workgraph's graph decorator still returns a plain TaskHandle, so when paired with node-graph > 0.6.5 every @task.graph handle loses build/run/run_get_graph/submit:

AttributeError: 'TaskHandle' object has no attribute 'build'

Change

Mirror the upstream pattern:

• New aiida_workgraph.task.GraphTaskHandle(TaskHandle), returned by the @task.graph decorator.
• build delegates to node-graph's GraphTaskHandle.build rather than inheriting it, because node-graph's __init__ takes spec only, while ours must also wire get_current_graph and graph_class=WorkGraph. The delegation only needs self._spec / self._graph_class / self.identifier, all present here.
• run, run_get_graph and submit move from TaskHandle onto GraphTaskHandle. They all materialize a WorkGraph via self.build, which only makes sense for graph specs; on plain task handles they only ever raised. Plain handles now keep node-graph's direct-execute run semantics, matching the behaviour of New syntax for the graph_builder's output #150

Compatibility

#150 is unreleased at the time of writing; the node-graph version pin should be bumped upon the next node-graph release, and this PR should not be merged prior.