Validate existing Docker network subnets

[aidankhogg]

Motivation

• Prevent silently reusing an existing Docker network that was created with a different IPAM subnet than the spec requests.

Description

• Inspect existing Docker network net.attrs["IPAM"]["Config"] and collect any configured Subnet values when client.networks.get(name) succeeds.
• Use ipaddress.ip_network(..., strict=False) to compare the configured subnet(s) to the requested subnet and allow reuse only when one matches exactly.
• Raise SubstrateError with a clear message listing configured subnet(s), the expected subnet, and remediation guidance when no matching subnet is found or IPAM config is missing.
• Added unit tests covering reuse on matching subnet, failure on mismatched subnet, and failure when IPAM config is absent.

Testing

• Ran pytest tests/test_m1_handlers.py::TestSubstrateHandler -q and all tests passed (11 passed).
• Ran formatting checks black netengine/handlers/substrate.py tests/test_m1_handlers.py and isort netengine/handlers/substrate.py tests/test_m1_handlers.py without changes needed.
• The new unit tests verify matching-subnet reuse, mismatched-subnet rejection, and missing-IPAM-config rejection and succeeded.

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Codex Task