[snmpagent][rfc1213] Optimize ipRouteNextHop default-route lookup

[vpandian-nokia]

What I did

Optimized ipRouteNextHop handling in rfc1213.py to avoid scanning all route keys in APPL_DB.
This preserves existing SNMP behavior for default-route next-hop reporting while reducing polling-path Redis/CPU overhead.

• Replaced ROUTE_TABLE:* scan + filter with direct lookup of ROUTE_TABLE:0.0.0.0/0.
• Kept behavior of selecting the first valid nexthop.
• Added robust parsing for nexthop tokens (strip() + invalid-token handling).
• Ensured route_list is updated only after successful nexthop parse.
  Updated unit test coverage in tests/test_rfc1213.py:
• Aligned tests to the new dbs_get_all code path.
• Added coverage for invalid first nexthop token with fallback to next valid token.
• Fixed Redis-exception test to mock the actual method used by NextHopUpdater (dbs_get_all).
• Added assertion that dbs_keys is not called in the optimized path.

How I did it

Modified NextHopUpdater.update_data() to:

1. Query only ROUTE_TABLE:0.0.0.0/0 from APPL_DB.
2. Read nexthop field from the returned route entry.
3. Parse nexthops left-to-right and pick the first valid IPv4 token.
4. Log warning for invalid tokens and continue scanning remaining tokens.
5. Populate SNMP route cache only when a valid nexthop is found.

How to verify it

1. Unit tests:

   • Updated/added unit test coverage in tests/test_rfc1213.py.

2. DUT functional checks:

   • No default route in APPL_DB -> SNMP ipRouteNextHop returns no instance.
   • Valid default route entry in APPL_DB -> SNMP returns expected nexthop.
   • Invalid first token in nexthop list -> warning logged; next valid nexthop is used.

3. Regression/stability:

   • Repeated polling shows stable behavior and no functional regressions.

Description for the changelog

Optimize SNMP ipRouteNextHop default-route lookup by replacing full route-table scan with direct ROUTE_TABLE:0.0.0.0/0 query and improving nexthop parsing robustness.

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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[vpandian-nokia]

@qiluo-msft This PR is a follow-up optimization of ipRouteNextHop (default-route-only) in PR #26.

Changes in this PR:

• replace ROUTE_TABLE:* scan with direct lookup of ROUTE_TABLE:0.0.0.0/0
• preserve existing default-route behavior
• add robustness for invalid nexthop tokens

Would appreciate your review when you have a chance.

[mssonicbld]

/azp run

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[vpandian-nokia]

@deepak-singhal0408: The relevant sonic-mgmt SNMP tests passed for this change. I would appreciate a reviewer taking a look and, if everything looks good, helping approve the PR.