Static L2 forwarding: per-VLAN mac_learning, BUM flood control, static FDB

[AnantKishorSharma]

What I did

Config-driven static L2 forwarding — wires three independent, per-VLAN, SAI-backed
sub-features from CONFIG_DB to the ASIC, each defaulting to today's behavior (no
functional change unless configured). Kept as three commits, one per sub-feature:

1. Per-VLAN MAC-learning disable — SAI_VLAN_ATTR_LEARN_DISABLE
2. Per-VLAN BUM flood disable — SAI_VLAN_ATTR_{UNKNOWN_UNICAST,UNKNOWN_MULTICAST,BROADCAST}_FLOOD_CONTROL_TYPE = NONE
3. Static FDB via CONFIG_DB — vlanmgr consumes the reserved FDB table → APPL FDB_TABLE (type=static); FdbOrch already programs SAI_FDB_ENTRY_TYPE_STATIC (unchanged).

Why I did it

None of these per-VLAN behaviors is configurable today (learning is per-bridge-port only;
per-VLAN flood control is set only internally by proxy_arp / VXLAN-EVPN; the CONFIG_DB FDB
table is reserved but has no consumer). The SAI attributes already exist, so the gap is the
CONFIG_DB schema + config→APPL plumbing. HLD: sonic-net/SONiC#2468.

How I verified it

• Mock (tests/mock_tests/portsorch_ut.cpp) + VS (tests/test_vlan.py, tests/test_fdb.py) tests.
• Hardware end-to-end: config apply-patch/gNMI → CONFIG_DB → APPL → ASIC verified for
  LEARN_DISABLE=true, the three *_FLOOD_CONTROL_TYPE=NONE, and SAI_FDB_ENTRY_TYPE_STATIC;
  plus L2 traffic (broadcast not flooded when disabled; unknown-unicast dropped without an
  entry, forwarded once a static FDB entry exists).

Details if related

Needs YANG sonic-net/sonic-buildimage#28545 and the config mac CLI sonic-net/sonic-utilities#4713.
Feature request: sonic-net/sonic-buildimage#28539.

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