fix(trimmer): trim last message when allowPartial: true and strategy: first

[schonmann]

Hello team! 👋

While consuming the trimMessages abstraction, I've noticed that for the following example:

const messages = [
  new SystemMessage(ENTITY_SUMMARIZER_PROMPT),
  new HumanMessage(
    `Please analyze this entity data: ${JSON.stringify(entity)}`
  ),
];

const trimmedMessages = await trimMessages({
  maxTokens: 128_000 * 0.9,
  strategy: 'first',
  allowPartial: true,
  textSplitter,
  tokenCounter,
}).invoke(messages, config);

The trimmedMessages would always strip away the HumanMessage completely – even though allowPartial is true, and we provided a textSplitter which splits text into small chunks.

Looking at the _firstMaxTokens code plus the equivalent Python algorithm, I saw a discrepancy in the way we're iterating. This PR fixes the issue by just navigating the entire array (including the last message).

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