fix(llm): accept nullable Anthropic input usage

[primorLee]

Issue for this PR

Closes #43759

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Accepts null for Anthropic's terminal input_tokens. Normalized usage keeps the earlier count from message_start, while raw provider metadata preserves the terminal null. Adds a regression test for both behaviors.

How did you verify your code works?

• bun test test/provider/anthropic-messages.test.ts --timeout 30000 from packages/llm — 29 passed
• bun typecheck from packages/llm — passed
• Prettier check passed for both changed files
• git diff --check passed

Screenshots / recordings

Not applicable.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• Not all checklist items are checked. Please confirm you have tested locally and have not included unrelated changes.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

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