Use Task.start_link in stream worker for caller propagation

[MikaAK]

Summary

LangEx.Graph.Stream.start_execution/3 spawns its worker via spawn_link/1, which does not propagate the parent's $callers / $ancestors process-dictionary entries. This breaks libraries that rely on per-PID ownership to route calls back to the test process, including:

• Ecto.Adapters.SQL.Sandbox.allow/3 / Mode.SandboxOwner when the worker touches a shared DB connection
• Custom per-PID test stubs (registries keyed on $callers)
• Caller-chain tracking for telemetry / tracing

Today the stream worker is invisible to these mechanisms, so tests that stub LLM responses per-PID silently fall through to the real HTTP layer (or fail with "no stub registered") when the code under test flows through Graphs.stream/3.

The fix

Swap spawn_link/1 for Task.start_link/1. Task already captures and restores $callers / $ancestors internally (that is what Task.Supervised.start_link does under the hood), and brings a few additional wins for free:

• OTP-style crash logging via Task's crash handler
• Correct participation in OTP telemetry / tracing
• No manual Process.put(:"$callers", ...) dance

 defp start_execution(graph, input, opts) do
   parent = self()
-
-  spawn_link(fn ->
-    state = State.apply_update(graph.initial_state, input, graph.reducers)
-    ...
-  end)
+
+  {:ok, pid} =
+    Task.start_link(fn ->
+      state = State.apply_update(graph.initial_state, input, graph.reducers)
+      ...
+    end)
+
+  pid
 end

Regression test

Added in test/lang_ex/graph/stream_test.exs: invokes the stream, has a node send its Process.get(:"$callers") / Process.get(:"$ancestors") back to the consumer, and asserts the consumer PID appears in both lists.

Compatibility

• No public API changes
• No new dependencies (Task is stdlib)
• Behaviour change is additive: consumers that do not rely on caller ownership see no difference
• Link semantics preserved — Task.start_link links to the calling process just like spawn_link

Testing

mix test
# 145 tests, 0 failures

[twist900]

@claude review