Introduced MAAS Networking Adapter and Contract-Based Config Rendering

[MoeSalah1999]

Proposed Commit Message

**Problem**

Cloud-init currently embeds platform-specific networking logic directly inside Distro.apply_network_config. This results in:

- High cross-platform coupling and conditional branching (if datasource == "MAAS": ...)
- Contributor friction: changes for one platform risk breaking others
- Mock-heavy tests that do not enforce platform contracts
- MyPy type errors due to apply() and apply_netplan() being referenced in adapters

**Solution**
This PR refactors MAAS networking into a dedicated adapter and introduces a render-only interface contract, leaving actual network application in Distro.apply_network_config.

Key changes:

1-Networking Adapter Interface:

- NetworkingAdapter defines a single abstract method render()
- Adapters only transform network config; they do not apply it
- Ensures type safety, mypy compliance, and clear platform contracts

2-MAAS Adapter:

- MAASNetworkingAdapter implements render()
- Encapsulates MAAS-specific network transformations (OVS bridge handling, MTU defaults)
- Drop-in replacement: does not change runtime behavior

3-Integration into apply_network_config():

- Optional adapter lookup via get_networking_adapter(self._datasource)
- Calls render() before parsing and writing network state
- Preserves bring_up semantics and existing renderer/activator behavior

4-Tests:

- Contract test verifies MAAS adapter implements render() and is a subclass of NetworkingAdapter
- Removed invalid apply() assertion in tests
- Platform-specific MAAS tests can verify rendered network config


**Motivation**

- Reduces cross-platform branching in core logic
- Encapsulates MAAS networking logic in a clear adapter
- Improves contributor confidence and CI feedback
- Provides a foundation for adding adapters for other cloud platforms

Fixes GH-6119

Additional Context

Next Steps:

1. Add adapters for EC2, Azure, GCE following the same contract
2. Refactor other platform-specific branches in apply_network_config
3. Add platform-specific golden tests for each adapter

I'll probably do this in a follow up PR.

Test Steps

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)