fix: stop leaking client supervisor children on failed and closed connections

[mvanhorn]

Summary

Short-lived client connections no longer leak GRPC.Client.Supervisor children. Orchestrator processes that fail to finalize a connection are terminated instead of being left under the DynamicSupervisor, and disconnected connections are fully cleaned up, so the supervisor's binary memory stays flat under connect/disconnect churn.

Why this matters

Reported in #531: a user running hundreds of short-lived connections (GRPC.Stub.connect/2 then GRPC.Stub.disconnect/1) on 1.0.0-rc.1 sees GRPC.Client.Supervisor top :recon.proc_count(:memory, 10) and :recon.bin_leak(10) with persistent growth; downgrading to 0.10.2 (no DynamicSupervisor in the client path) eliminates it, and a collaborator confirmed the diagnosis and labeled it a bug. The regression is in the 0.11+/1.0 client connection architecture: GRPC.Client.Connection.connect/2 leaves the orchestrator child running when finalize_connection/2 returns {:error, :no_connection}, and the disconnect path leaves per-connection state behind.

Changes

• connect/2 now terminates the orchestrator child when connection finalization fails, so error paths cannot accumulate supervised processes.
• The disconnect path releases the connection's supervised resources instead of leaving the child alive under GRPC.Client.Supervisor.

Testing

Added regression tests that drive repeated connect/disconnect cycles and connect-failure cycles, asserting DynamicSupervisor.count_children/1 on GRPC.Client.Supervisor returns to baseline after each cycle. Ran the client connection test files with mix test.

Fixes #531

[sleipnir]

@mvanhorn Some tests are failing, could you please check them?

[mvanhorn]

The OTP-matrix failures were the connection tests asserting active == 0 on the shared GRPC.Client.Supervisor — under the full suite other tests leave children behind (CI showed active: 44), so the assertions failed even though the lifecycle fix works. 5a6f752 captures the supervisor's pre-existing child count in setup and asserts the count returns to that baseline instead. All 14 connection tests pass locally.