…ma (#12471)

## Background

PR #12384 introduced support for Anthropic's compaction feature, but the
streaming Zod schema for compaction_delta events requires content to be
a non-nullable string. Anthropic's API sends compaction_delta events
with content: null (e.g. the initial frame before the compaction summary
text), which causes Zod validation to fail at runtime.

## Summary

Changed content: z.string() to content: z.string().nullish() in the
compaction_delta streaming schema in
packages/anthropic/src/anthropic-messages-api.ts. This matches the
existing content_block_start schema for compaction, which already uses
.nullish().

## Manual Verification

Reproduction requires streaming a conversation that exceeds the
compaction trigger threshold (50k+ tokens) with @ai-sdk/anthropic, which
triggers a compaction_delta event with content: null. This is not
practical to verify without an API key and a large conversation, but the
added regression test simulates this exact scenario by streaming a
compaction_delta chunk with content: null.

## Checklist

- Tests have been added / updated (for bug fixes / features)
- Documentation has been added / updated (for bug fixes / features)
- A patch changeset for relevant packages has been added (for bug fixes
/ features - run pnpm changeset in the project root)
- I have reviewed this pull request (self-review)

## Related Issues

Fixes #12470

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Co-authored-by: Embedder <215220128+embedder-dev@users.noreply.github.com>
Co-authored-by: Aayush Kapoor <83492835+aayush-kapoor@users.noreply.github.com>