fix: traffic split plugin not validating upstream_id

[shreemaan-abhishek]

Description

Fixes #9996

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[jiangfucheng]

@shreemaan-abhishek Please take a look the failed CI.

[Nobilta]

@shreemaan-abhishek @jiangfucheng Hi,everyone,when i test this function in apisix3.5,this error is still exist

[Nobilta]

@shreemaan-abhishek @jiangfucheng Hi,everyone,when i test this function in apisix3.5,this error is still exist

In both the patch and put methods

[Nobilta]

@shreemaan-abhishek @jiangfucheng we can modify check_schema function in traffic-split.lua and add the validating function.

function _M.check_schema(conf)
    local ok, err = core.schema.check(schema, conf)
    if not ok then
        return false, err
    end
    if conf.rules then
        for _, rule in ipairs(conf.rules) do
            if rule.match then
                for _, m in ipairs(rule.match) do
                    local ok, err = expr.new(m.vars)
                    if not ok then
                        return false, "failed to validate the 'vars' expression: " .. err
                    end
                end
            end
            if rule.weighted_upstreams then
                for _, wupstream in ipairs(rule.weighted_upstreams) do
                    local ups_id = wupstream.upstream_id
                    if ups_id then
                        local ups = upstream.get_by_id(ups_id)
                        core.log.error("upstream:", ups)
                        if not ups then
                            return false, "failed to fetch upstream info by "
                                        .. "upstream id: " .. ups_id
                        end
                    end
                end
            end
        end
    end
    return true
end