[Nexthop][fboss2-dev] Add fboss2-dev delete load-balancing subcommands

[vybhav-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

What: Adds fboss2-dev delete load-balancing ecmp and delete load-balancing lag, removing the matching LoadBalancer entry from sw.loadBalancers. Errors out if the entry is not configured.

Why: The config side (config load-balancing ecmp|lag <attr> <value>) can create and mutate load-balancer entries but nothing can remove one; an empty loadBalancers list is the valid unconfigured state.

How: New CmdDeleteLoadBalancing parent + ecmp/lag leaf handlers sharing a removeLoadBalancer() helper. Removal commits hitlessly — SaiSwitch dispatches LoadBalancersDelta removals to SaiSwitchManager::removeLoadBalancer

The integration test covers ecmp delete + restore end-to-end. LAG delete shares the same implementation and is covered by unit tests

Test Plan

Unit (6 new for CmdDeleteLoadBalancing, full suite green):

[==========] 719 tests from 59 test suites ran. (50555 ms total)
[  PASSED  ] 719 tests.

Integration on a Nexthop device (delete → verify gone → restore via config CLI → verify match, all hitless):

[ RUN      ] DeleteLoadBalancingTest.DeleteThenRestoreEcmp
[       OK ] DeleteLoadBalancingTest.DeleteThenRestoreEcmp (236 ms)
[  PASSED  ] 1 test.

Sample usage on the device:

$ fboss2-dev delete load-balancing ecmp
Deleted ecmp load-balancer
$ fboss2-dev config session diff
--- current live config
+++ session config
@@ -955,26 +955,6 @@
-      {
-        "algorithm": 1,
-        "fieldSelection": { ... },
-        "id": 1
-      }
$ fboss2-dev config session commit
Config session committed successfully as dbb1b8e and config reloaded for fboss_sw_agent.
$ fboss2-dev delete load-balancing ecmp
localhost: Invalid argument: No ecmp load-balancer configured

Review Findings

Pre-publication review (11-reviewer sweep + verifier, confidence >= 0.7). Two findings, both fixed in this PR before publication:

File	Severity	Issue	Resolution
DeleteLoadBalancingTest.cpp	HIGH	The delete was committed before the restore ran, and every restore step was a fatal ASSERT_TRUE. A mid-restore failure left the device with no ECMP load-balancer, since the failure path only discards the uncommitted session.	Restore logic extracted into stageEcmpRestore(); the test arms a pending restore before the delete commit and clears it after the restore commits, with a TearDown() that puts the entry back if the test aborted in between.
LoadBalancingTestUtils.h	MEDIUM	Reverse mappings switched on raw integer literals for thrift enum values that have names.	Switched to cfg::HashingAlgorithm / cfg::IPv4Field / cfg::IPv6Field / cfg::TransportField / cfg::MPLSField. This also splits the ipv4/ipv6 handling, which previously shared a branch that could return flow-label for an ipv4 field.