Surface auto_start on ez.run()

[cboulay]

Summary

Exposes the existing auto_start option on the public ez.run() / GraphRunner API.

The auto_start mechanism already exists lower in the stack — GraphContext(auto_start=...) → GraphServer.ensure(auto_start=...) decides whether to spawn a GraphServer when none is reachable at a given graph_address, and it's already plumbed through sync.py and hotpath.py. But ez.run() had no way to pass it through, so calling run() with an explicit graph_address would refuse to start a server even when the caller wanted one.

This adds auto_start: bool | None to GraphRunner.__init__ and run(), threads it into create_graph_context(), and documents the semantics.

Motivation

Lets a parent process pick an ephemeral port and hand it to a child that owns the graph: the child binds to that explicit address and passes auto_start=True so it spawns the server rather than expecting one to already exist.

Behavior

• auto_start=None (default): unchanged — a server is auto-started only when no explicit graph_address is given and no environment override is set.
• auto_start=True: force auto-start even when binding to a specific address.

Tests

Adds test_run_auto_start_with_explicit_address, which runs a pub/sub graph on an explicit ephemeral port with auto_start=True and asserts all messages are delivered.

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