Bug to handle duplicate subnet across different vpc networks

##### SUMMARY

GCP doesn't allow to create any subnet with the same name in the same region if already exists in any other VPC network, and the original module doesn't throw error. This PR suggests adding a condition check and error out for this scenario.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME

plugins/modules/gcp_compute_subnetwork.py

##### ANSIBLE VERSION

```paste below
ansible [core 2.14.2]
config file = /Users/x/proj/hands-on/ansible/sbn-bug-ansible/playbooks/ansible.cfg
configured module search path = ['/Users/x/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
ansible python module location = /usr/local/lib/python3.9/site-packages/ansible
ansible collection location = /Users/x/.ansible/collections:/usr/share/ansible/collections
executable location = /usr/local/bin/ansible
python version = 3.9.16 (main, Dec 7 2022, 10:16:11) [Clang 14.0.0 (clang-1400.0.29.202)] (/usr/local/opt/python@3.9/bin/python3.9)
jinja version = 3.0.1
libyaml = True

```

##### COLLECTION VERSION

```paste below
nothing special, just local ansible.cfg
```

##### CONFIGURATION

```paste below
ansible [core 2.14.2]
```

##### OS / ENVIRONMENT

MacOS 13.0 (22A380)

##### STEPS TO REPRODUCE

#A1: create a network vpc-a
#A2: create a subnetwork subnet-a in vpc-a
#B1: create another network vpc-b
#B2: create a same-name subnetwork subnet-a in vpc-b

```yaml
---
- name: Define variables
set_fact:
project: your-project-name
region: us-east4
gcp_auth_kind: application

- name: "#A1: create a network vpc-a"
google.cloud.gcp_compute_network:
name: "vpc-a"
auto_create_subnetworks: 'false'
project: "{{ project }}"
auth_kind: "{{ gcp_auth_kind }}"
state: present
register: network_a

- name: "#A2: create a subnetwork subnet-a in vpc-a"
google.cloud.gcp_compute_subnetwork:
name: "subnet-a"
region: "{{ region }}"
network: "{{ network_a }}"
ip_cidr_range: 172.16.0.0/20
project: "{{ project }}"
auth_kind: "{{ gcp_auth_kind }}"
state: present

- name: "#B1: create another network vpc-b"
google.cloud.gcp_compute_network:
name: "vpc-b"
auto_create_subnetworks: 'false'
project: "{{ project }}"
auth_kind: "{{ gcp_auth_kind }}"
state: present
register: network_b

- name: "#B2: create a same name subnetwork subnet-a in vpc-b"
google.cloud.gcp_compute_subnetwork:
name: "subnet-a"
region: "{{ region }}"
network: "{{ network_b }}"
ip_cidr_range: 172.16.0.0/20
project: "{{ project }}"
auth_kind: "{{ gcp_auth_kind }}"
state: present
```

##### EXPECTED RESULTS

Task #B2 should throw an error saying the same name subnet already exists in another VPC network (vpc-a).

##### ACTUAL RESULTS

The original module finds the subnet and shows it on a different VPC network (vpc-a), and continues to call the Update() function and actually does nothing. No any error and no any subnet created at vpc-b.

```paste below
changed: [localhost] => {
"changed": true,
"creationTimestamp": "2023-03-02T05:37:59.024-08:00",
"fingerprint": "HQ6lQkX4WFI=",
"gatewayAddress": "172.16.0.1",
"id": "8170811963898402904",
"invocation": {
"module_args": {
"auth_kind": "application",
"description": null,
"env_type": null,
"ip_cidr_range": "172.16.0.0/13",
"name": "subnet-a",
"network": {
"ansible_facts": {
"discovered_interpreter_python": "/usr/local/bin/python3.11"
},
"autoCreateSubnetworks": false,
"changed": false,
"creationTimestamp": "2023-03-01T08:54:08.590-08:00",
"failed": false,
"id": "2564930274501737951",
"kind": "compute#network",
"name": "vpc-b",
"networkFirewallPolicyEnforcementOrder": "AFTER_CLASSIC_FIREWALL",
"routingConfig": {
"routingMode": "REGIONAL"
},
"selfLink": "https://www.googleapis.com/compute/v1/projects/xx-demo/global/networks/vpc-b",
"selfLinkWithId": "https://www.googleapis.com/compute/v1/projects/xx-demo/global/networks/2564930274501737951",
"subnetworks": [
],
"warnings": [
..
]
},
"private_ip_google_access": false,
"private_ipv6_google_access": null,
"project": "xx-demo",
"region": "us-east4",
"scopes": [
"https://www.googleapis.com/auth/compute"
],
"secondary_ip_ranges": null,
"service_account_contents": null,
"service_account_email": null,
"service_account_file": null,
"state": "present"
}
},
"ipCidrRange": "172.16.0.0/14",
"kind": "compute#subnetwork",
"name": "subnet-a",
"network": "https://www.googleapis.com/compute/v1/projects/xx-demo/global/networks/vpc-a",
"privateIpGoogleAccess": false,
"privateIpv6GoogleAccess": "DISABLE_GOOGLE_ACCESS",
"purpose": "PRIVATE",
"region": "https://www.googleapis.com/compute/v1/projects/xx-demo/regions/us-east4",
"selfLink": "https://www.googleapis.com/compute/v1/projects/xx-demo/regions/us-east4/subnetworks/subnet-a",
"stackType": "IPV4_ONLY"
}
```

Start with plugins/modules/gcp_compute_subnetwork.py and reproduce the four-step playbook using two VPC networks in the same project and region. Trace how the existing subnetwork is found and confirm that the completed change reports an error when the requested name already belongs to another VPC network, rather than updating it or reporting changed.

Written by the indexing model from the issue text.