Fix agent stream callback

[devinyf]

I did a little change in this PR to handled 2 problems when using AgentFinalStreamHandler:

I followed this example to study using agent
https://github.com/tmc/langchaingo/blob/main/examples/mrkl-agent-example/mrkl_agent.go
and use a handler to get streaming result like this:

	callbackHandler := callbacks.NewFinalStreamHandler()
	// callbackHandler.PrintOutput = true

	executor, err := agents.Initialize(
		llm,
		agentTools,
		agents.ZeroShotReactDescription,
		agents.WithMaxIterations(5),
		agents.WithCallbacksHandler(callbackHandler),
	)

problem1. Detect keyword failed:

reason: cut out slice handler.LastTokens before doing detect keyword will cause Detection can’t success

for example:
before the handler.LastTokens string value is

"result Final Answer: As of February 18,"

after the reassign the slice value, it's become

"Answer: As of February 18,"

Then it can't pass the keywords check process which should contained Final Answer: keyword in LastTokens.

So I changed code order, check keywords first, then reassign slice

problem2. Final Answer sometimes contain colon symbol : at the beginning

Add few code to filter the colon if it exist

May this be a positive contribution.

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[tmc]

can you flesh out some more expected cases and use a table driven test in the test method?

[devinyf]

I've updated test case in this PR and #612 , Both using table driven

[tmc]

please place this at the top of the test (after t.Parallel()) and approach it like:

cases := []struct { ...

[devinyf]

updated all test case inline with:

cases := []struct { ...

please review

[tmc]

LGTM