Fix googleai malformed function call crash

[nelsonkopliku]

When Gemini (tested with gemini-2.5-flash) cannot form a valid function call, it returns a candidate with the following shape:

%{
  "finishMessage" => "Some message...",
  "finishReason" => "MALFORMED_FUNCTION_CALL",
  "index" => 0
}

This in turn ends up in the last do_process_response match

langchain/lib/chat_models/chat_google_ai.ex

Line 902 in 8a5c2e6

def do_process_response(_model, other, _) do

and returns an {:error, %LangChainError{}} tuple and making the deltas reindexing reduction with a ** (KeyError) key :index not found in...

This PR makes sure the streaming does not crash and bubbles up the (first encoundered) error.

Caveats:

• could be a model specific behaviour, newer models did not present the issue AFAICS
• for completeness, the test covers for cases where the response body is a mix of successful MessageDeltas and error tuples, although IRL testing always manifested with a single entry list with the error tuple
• depending on whether the "mix of successful MessageDeltas and error tuples" case is real, there might be different considerations to make like:
  • does it make sense to return the first encountered error? what about the others?
  • what about the non-errored MessageDeltas? would it be valuable to not "miss" in the best effort failure handling?
  • in case of many errored candidates, would it be useful to accumulate all of them in a single LangChainError struct that contains a list of many LangChainError?

I don't have evidences, yet, about other cases so I went for the simplest and less intrusive approach.

[Copilot]

Pull request overview

This PR addresses a crash in ChatGoogleAI streaming mode when Gemini returns candidates with finishReason: "MALFORMED_FUNCTION_CALL" (or other unexpected candidate shapes), ensuring streaming returns an error tuple instead of raising during delta reindexing.

Changes:

• Detect {:error, %LangChainError{}} entries in streamed candidate deltas and return the first error early (avoids KeyError on :index during reindexing).
• Extract the delta reindexing logic into a dedicated reindex_deltas/1 helper.
• Add a test covering a streamed response body containing a mix of deltas and error tuples.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/chat_models/chat_google_ai.ex	Prevents streaming reduce/reindex from crashing when an error tuple appears among streamed candidates; refactors reindexing into a helper.
test/chat_models/chat_google_ai_test.exs	Adds regression test ensuring streaming returns an error instead of crashing when malformed-function-call candidates appear.

Comments suppressed due to low confidence (1)

test/chat_models/chat_google_ai_test.exs:2036

• Same as above: streamed %MessageDelta{} from ChatGoogleAI uses a single %ContentPart{} (or nil) for content, not a list. Aligning this fixture with actual streaming output will keep the test representative and reduce the risk of false confidence.

      delta2 = %LangChain.MessageDelta{
        content: [%LangChain.Message.ContentPart{type: :text, content: "Part 2"}],
        index: 0,
        role: :assistant,
        status: :incomplete
      }

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[nelsonkopliku]

Hey @brainlid, first off thanks for this library and for accepting my previous, and first contribution!

I am wondering whether this PR is making sense for users of the lib from your perspective - I at least encountered the issue.

A note: I have another fix about google ai here nelsonkopliku#3.
I will wait for this one to go through before opening that one.

[brainlid]

Thanks @nelsonkopliku!
❤️💛💙💜