networkconnectivity: fix hub-only google_network_connectivity_transport (exactly one of hub/network)

[AzySir]

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Description

google_network_connectivity_transport treated network as required, so NCC hub-only configs failed provider validation even though the API allows either network or hub.

This change:

• removes required: true from network
• adds exactly_one_of: [hub, network] on both fields
• updates field descriptions
• adds a unit test that asserts the schema shape

Fixes hashicorp/terraform-provider-google#28425

Testing

• Generated beta provider for networkconnectivity / Transport
• Unit test: TestUnitNetworkConnectivityTransport_networkHubExactlyOneOf (PASS)

networkconnectivity: fixed `google_network_connectivity_transport` incorrectly requiring `network` when `hub` is set; exactly one of `network` or `hub` is required (beta)

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[github-actions]

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[AzySir]

@SirGitsalot hope you're well - can we get a review on this one please