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

Previously, an invalid minimumCapacity in an aggregate port config would trip a raw CHECK in computeMinimumLinkCount, aborting the agent with SIGABRT:

F20260908 17:41:52.271183 18014 ApplyThriftConfig.cpp:3611] Check failed: minCapacity.get_linkPercentage() <= 1 (50 vs. 1)

This PR replaces those CHECKs with FbossError throws so invalid config is rejected with a clear, actionable message instead of crashing.

Test Plan

Unit tests

Added AggregatePort.invalidMinimumCapacityLinkPercentageThrows and AggregatePort.invalidMinimumCapacityLinkCountThrows in AggregatePortTests.cpp, asserting publishAndApplyConfig throws FbossError for out-of-range linkPercentage (50) and linkCount (0).

On-device verification

1. Boot path — clear FATAL message instead of raw CHECK

Running the fixed fboss_sw_agent against a config with minimumCapacity.linkPercentage = 50:

F0908 18:12:48.950331 24000 SwAgentInitializer.cpp:116] switch initialization failed: facebook::fboss::FbossError: Minimum capacity linkPercentage must be in (0, 1], got 50

2. Runtime reload path — config rejected, agent stays alive

With the agent booted on a valid config, restoring the invalid linkPercentage: 50 on disk and running fboss2-dev config reload hitless:

 CLI output (exit 1):
 localhost: Thrift call failed: 'facebook::fboss::FbossError: Minimum capacity linkPercentage must be in (0, 1], got 50'

Agent log:

 V0908 18:17:34.977625 24260 ThriftHandler.cpp:2755] reloadConfig thrift request received from ::1 (unknown)
 V0908 18:17:34.978168 24193 SwSwitch.cpp:1993] preparing state update reload config initiated by thrift call; # Pending updates 0
 V0908 18:17:34.979446 24260 ThriftHandler.cpp:2755] reloadConfig thrift request failed in 1ms
 E20260908 18:17:34.979475 24210 service_tcc.h:152] Service handler threw an uncaught exception in method reloadConfig: facebook::fboss::FbossError: Minimum capacity linkPercentage must be in (0, 1], got 50.

Agent survived the rejection — same PID before and after, service still active:

PID before reload: 24161
PID after reload: 24161
fboss_sw_agent.service: active