Add sonic validate command for YANG-based config_db.json validation

[berendt]

Validates generated SONiC ConfigDB JSON against the bundled SONiC YANG models so config drift, type errors and broken leafrefs are caught before configs are pushed to switches. Sources can be a local file, NetBox local_context, the export directory or a fresh in-memory generation.

Pulls in sonic-yang-mgmt as a VCS install (not on PyPI), pinned to sonic-buildimage release branch 202511. Renders the three previously missing YANG models (sonic-extension, sonic-types, sonic-policer) from their upstream Jinja templates so the bundled model set is loadable by libyang.

AI-assisted: Claude Code

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="osism/commands/sonic.py" line_range="1353-1354" />
<code_context>
+                f"(supported: {', '.join(SUPPORTED_HWSKUS)})."
+            )
+            return None
+        config_version = None
+        if device.custom_fields.get("sonic_parameters", {}).get("config_version"):
+            config_version = device.custom_fields["sonic_parameters"]["config_version"]
+        return generate_sonic_config(device, hwsku, None, config_version)
</code_context>
<issue_to_address>
**issue (bug_risk):** Access to device.custom_fields can raise AttributeError when the attribute is missing.

In `_config_from_generate`, this block assumes `device.custom_fields` always exists:

```python
config_version = None
if device.custom_fields.get("sonic_parameters", {}).get("config_version"):
    config_version = device.custom_fields["sonic_parameters"]["config_version"]
```

If a device lacks `custom_fields`, this will raise `AttributeError` instead of using your existing guarded logic. Please either reuse the earlier `hasattr(device, "custom_fields")` check here, or factor out something like `sonic_params = getattr(device, "custom_fields", {}).get("sonic_parameters", {})` and use it for both `hwsku` and `config_version` to keep error handling consistent.
</issue_to_address>

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[sourcery-ai]

issue (bug_risk): Access to device.custom_fields can raise AttributeError when the attribute is missing.

In _config_from_generate, this block assumes device.custom_fields always exists:

config_version = None
if device.custom_fields.get("sonic_parameters", {}).get("config_version"):
    config_version = device.custom_fields["sonic_parameters"]["config_version"]

If a device lacks custom_fields, this will raise AttributeError instead of using your existing guarded logic. Please either reuse the earlier hasattr(device, "custom_fields") check here, or factor out something like sonic_params = getattr(device, "custom_fields", {}).get("sonic_parameters", {}) and use it for both hwsku and config_version to keep error handling consistent.