callbacks: supplement single test coverage for CombiningHandler

[flc1125]

Ex:

package main

import (
	"context"

	"github.com/tmc/langchaingo/callbacks"
)

type MyCustomHandler struct {
	callbacks.SimpleHandler
}

func main() {
	h := callbacks.NewCombiningHandler(
		&callbacks.SimpleHandler{},
		&callbacks.LogHandler{},
		&MyCustomHandler{},
	)

	h.HandleText(context.Background(), "Hello, world!")
}

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.