feat(anthropic): support cache diagnostics

[iroiro147]

Summary

• add cacheDiagnostics.previousMessageId for Anthropic provider requests and map it to diagnostics.previous_message_id
• automatically enable the cache-diagnosis-2026-04-07 beta when cache diagnostics are used
• expose Anthropic cache diagnostics response metadata for generate and stream responses
• keep Google Vertex Anthropic from sending unsupported cache diagnostics beta/body fields

Fixes #15986.

Tests

• pnpm --filter @ai-sdk/anthropic exec vitest run --config vitest.node.config.js src/anthropic-language-model.test.ts -t "cache diagnostics" (5 passed)
• pnpm --filter @ai-sdk/anthropic type-check
• pnpm --filter @ai-sdk/anthropic test:node (448 passed)
• pnpm --filter @ai-sdk/anthropic test:edge (448 passed)
• pnpm --filter @ai-sdk/anthropic build
• pnpm --filter @ai-sdk/google-vertex exec vitest run --config vitest.node.config.js src/anthropic/google-vertex-anthropic-provider.test.ts (13 passed)
• pnpm --filter @ai-sdk/google-vertex type-check
• pnpm exec oxfmt --check on touched files
• pnpm exec oxlint on touched TypeScript files
• pnpm changeset status --since origin/main
• git diff --cached --check