Add dynamic client support (PIDs, via tuples, global tuples)

[montebrown]

Enable MCP adapters to work with dynamically started clients instead of only module names. This unlocks per-job client patterns needed for scenarios like Playwright browser automation where each job requires an isolated browser context to prevent race conditions between concurrent jobs.

Why this matters:

• Shared MCP clients cause race conditions.
• Single persistent connections defeat load balancer distribution.
• Per-job clients enable true horizontal scaling across server pools.
• Memory stays bounded (contexts cleaned up when jobs complete).

Changes:

• Config: Accept PIDs (with alive check), {:via, Registry, _}, {:global, _}
• ToolExecutor: Dispatch based on client type, resolve supervisor PIDs to Anubis.Client.Base automatically
• TestSupport: Use apply/3 for Mimic calls to avoid compile-time dep
• README: Document "Client Types" and "Dynamic Clients" sections

Example usage: {:ok, client_pid} = MyApp.MCPClient.start_link(opts) :ok = Adapter.wait_for_server_ready(client_pid) adapter = Adapter.new(client: client_pid)
#... use adapter ...
Supervisor.stop(client_pid)