sonic: Generate the entries of the default VRF

[berendt]

Closes #2515

Since the table-ownership enforcement, generate_sonic_config() drops the content of every owned table up front and rebuilds it. The default-VRF entries that the base config_db.json shipped (#1937) were never regenerated, because the generator only writes these tables for the VRFs NetBox carries.

Change

The generator now owns the default-VRF entries as hardcoded SONiC policy, with the values the base config used to carry:

• VRF|default
• BGP_GLOBALS|default attributes (network_import_check, ebgp_requires_policy, holdtime/keepalive, …), with router_id/local_asn merged into the entry instead of replacing it
• BGP_GLOBALS_AF default|ipv4_unicast, default|ipv6_unicast, default|l2vpn_evpn
• BGP_GLOBALS_ROUTE_ADVERTISE default|L2VPN_EVPN|IPV4_UNICAST and …|IPV6_UNICAST
• ROUTE_REDISTRIBUTE default|connected|bgp|ipv4 and default|connected|bgp|ipv6

The entries are dropped from files/sonic/config_db.json. Since VRFs with a VNI copy their BGP_GLOBALS entry from the default VRF, they pick up the full attribute set again as well.

All five tables are now written by both the orchestrator (default VRF) and _add_vrf_configuration (per-VRF entries), so they join MULTI_OWNER_OWNED_TABLE_KEYS; the existing static guard keeps both writers on the per-key merge pattern. _add_vrf_configuration creates ROUTE_REDISTRIBUTE via setdefault instead of rebinding it.

Beyond the three BGP_GLOBALS* tables named in the issue, VRF|default and the default-VRF ROUTE_REDISTRIBUTE entries were lost in exactly the same way, so they are covered here too. The only other owned table with base content is SNMP_SERVER|SYSTEM, which _add_snmp_configuration already regenerates.

Tests

• Regression test: a device without VNI VRFs gets all default-VRF entries, asserted with literal values
• The policy constants are deep-copied per device, so one device's config cannot rewrite the policy for the next
• Completeness test: every policy entry reaches the config under the expected key
• The ownership tests were extended/adjusted: a stale BGP_GLOBALS_AF|default|ipv4_unicast from the base config is overwritten by the generated policy, and a device without a primary IP now gets the policy without router_id/local_asn

[sourcery-ai]

Hey - I've left some high level feedback:

• The DEFAULT_VRF_* policy dictionaries are mutable module-level globals; consider wrapping them in immutable structures (e.g. MappingProxyType or dataclasses with frozen=True) to prevent accidental in-place modification elsewhere in the codebase.
• The default VRF emission logic added to generate_sonic_config() is fairly substantial; factoring it into a small helper (e.g. _add_default_vrf_configuration(config, device)) would keep the main generator function easier to scan and isolate the default-VRF behavior.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The DEFAULT_VRF_* policy dictionaries are mutable module-level globals; consider wrapping them in immutable structures (e.g. MappingProxyType or dataclasses with frozen=True) to prevent accidental in-place modification elsewhere in the codebase.
- The default VRF emission logic added to generate_sonic_config() is fairly substantial; factoring it into a small helper (e.g. _add_default_vrf_configuration(config, device)) would keep the main generator function easier to scan and isolate the default-VRF behavior.

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