feat: add variadic function support to eval

[icehaunter]

Summary

• add variadic function support to the eval stack, including defpostgres definitions and overload lookup support
• lower coalesce, greatest, and least through the shared function parsing and execution path
• tighten variadic parsing behavior around explicit VARIADIC calls and precedence between exact and variadic overloads
• implement true coalesce short-circuiting in parser reduction and runtime execution

Testing

• mix test test/electric/replication/eval_test.exs test/electric/replication/eval/*.exs

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
2667	1	2666	1

View the top 1 failed test(s) by shortest run time

Elixir.Electric.Replication.PublicationManagerTest::test component restarts handles relation tracker restart

Stack Traces | 0.16s run time

22) test component restarts handles relation tracker restart (Electric.Replication.PublicationManagerTest)
     .../electric/replication/publication_manager_test.exs:503
     ** (exit) exited in: GenServer.call({:via, Registry, {:"Electric.ProcessRegistry:Electric.Replication.PublicationManagerTest test component restarts handles relation tracker restart", {Electric.Replication.PublicationManager.RelationTracker, nil}}}, {:remove_shape, "45463459-1775846139201787"}, 5000)
         ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
     code: PublicationManager.remove_shape(ctx.stack_id, shape_handle)
     stacktrace:
       (elixir 1.19.5) lib/gen_server.ex:1135: GenServer.call/3
       (electric 1.5.1) .../replication/publication_manager/relation_tracker.ex:73: Electric.Replication.PublicationManager.RelationTracker.remove_shape/2
       .../electric/replication/publication_manager_test.exs:522: (test)

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