[Refactor] Roll out stream helpers to Anthropic Vertex spec

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Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Migrated anthropic-vertex.spec.ts to the shared stream helpers.
• Replaced array-driven for (const chunk of mockStream) yield chunk iterators, direct-yield stream mocks, and default streaming mocks with asyncStreamFrom.
• Replaced all pure collect loops and consume/drain loops with collectStream, including the rejection-drain assertion.
• Removed the now-unused ApiStreamChunk import after the migration.
• Left the existing ESLint suppressions count unchanged after verification.

Why this change was made

This is the more ambitious continuation of the approved test-utils rollout: one of the largest remaining provider-spec stream-mock files, cleared in a single focused PR.

Impact

No user-facing behavior change. The 44-test Anthropic Vertex spec now uses the shared stream-helper idiom, targeted tests passed, src lint/typecheck passed, and the full Zoo-Code test suite passed.

Related PRs

• Async-stream test-utils pilot: [Refactor] Pilot shared stream test utilities #1086
• OpenAI-family stream-helper batch: [Refactor] Batch roll out stream helpers to OpenAI-family specs #1091
• Base OpenAI stream-helper batch: [Refactor] Roll out stream helpers to base OpenAI specs #1092
• OpenAI spec stream-helper migration: [Refactor] Roll out stream helpers to OpenAI spec #1093

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1 issue outstanding. See task

• src/api/providers/__tests__/anthropic-vertex.spec.ts:9 — ApiStreamChunk is still imported but no longer used anywhere in the file after the collectStream migration (all typed chunks: ApiStreamChunk[] declarations were removed). Lint/typecheck don't catch it since unused-var checks are off, but the sibling OpenAI spec migration ([Refactor] Roll out stream helpers to OpenAI spec #1093) dropped the equivalent import. Remove the import to keep the migration complete.

_{Reviewed 5a7f681}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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Addressed the outstanding review finding in anthropic-vertex.spec.ts by removing the unused ApiStreamChunk import.

• Commit: 0cbe77fbda650d578593b819db90a7a520431814
• Validation: targeted Anthropic Vertex spec, check-types, and lint passed before push; PR metadata refreshed.
• Thread state: no inline review thread existed for this finding; the fix is reflected in the updated PR body.