Adapt SNMP for state_db mgmt_port_table entries

[rsh2prasad]

Why I did it

ifHighSpeed (ifMIB.ifXTable.ifHighSpeed, OID .1.3.6.1.2.1.31.1.1.1.15) returns 0 (or the 40000 default) for management interfaces in setups where the operational speed of the mgmt port is published in STATE_DB rather than CONFIG_DB.

Today InterfaceMIBUpdater._get_if_entry() looks up management ports under MGMT_PORT|<name> in CONFIG_DB only. On platforms where the mgmt driver writes the live link speed (and other operational fields) into STATE_DB's MGMT_PORT_TABLE|<name>, the CONFIG_DB row may not carry a speed field at all, so ifHighSpeed falls back to the hard-coded default and SNMP polling shows the wrong management-port speed.

rfc1213.py already has the same two-tier lookup helper (mgmt_if_entry_table_state_db) for ifSpeed. This PR brings rfc2863.py (ifXTable) in line with that behavior so ifHighSpeed reflects reality on those platforms.

What I did

• Added a new InterfaceMIBUpdater._get_if_entry_state_db() helper in src/sonic_ax_impl/mibs/ietf/rfc2863.py. For an OID that maps to a management port, it reads the entry from STATE_DB via the existing mibs.mgmt_if_entry_table_state_db(name) table builder (MGMT_PORT_TABLE|<name>), using Namespace.dbs_get_all(..., blocking=False) so a missing key returns None instead of stalling the SNMP request.
• Updated get_high_speed() so that when the queried OID is a management port, it pulls the entry from STATE_DB (via the new helper); for all other interface types (physical ports, LAGs, VLANs) the existing CONFIG_DB/APPL_DB-based _get_if_entry() path is unchanged.
• Behavior is purely additive: existing flows for non-mgmt ports are untouched, and int(entry.get("speed", 40000)) still applies if STATE_DB has no speed field — so the default fallback is preserved.

Files changed

• src/sonic_ax_impl/mibs/ietf/rfc2863.py — new _get_if_entry_state_db() and updated get_high_speed().
• tests/test_hc_interfaces.py — four new unit tests covering:
  • GETNEXT walks over ifXTable end up at the mgmt-port row (proves mgmt port is reachable in the MIB).
  • GET ifHighSpeed on a mgmt port returns the value sourced from STATE_DB (e.g. 2000).
  • GETNEXT ifHighSpeed on the prior OID lands on the mgmt port with the correct speed.
  • GETNEXT past the last mgmt port returns END_OF_MIB_VIEW.
• tests/mock_tables/state_db.json, tests/mock_tables/global_db/state_db.json, tests/mock_tables/asic0/state_db.json, tests/mock_tables/asic1/state_db.json — extended MGMT_PORT_TABLE mocks with description, alias, admin_status, speed, etc., including a NotExistInStateDB case to exercise the missing-row path. Multi-asic mocks (asic0 / asic1) use the MGMT_PORT_TABLE:<name> colon-separated form to match how multi-asic STATE_DB serializes keys.

[mssonicbld]

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[mssonicbld]

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