fix(provider/anthropic): preserve streamObject deltas with thinking

[giulio-leone]

Summary

• emit text-delta for JSON tool outputs when Anthropic sends full JSON only in content_block_start.input
• keep existing input_json_delta behavior, but fall back to start-input emission when deltas are absent
• add regression coverage for thinking + structured output (jsonTool) streaming path

Validation

• pnpm --filter @ai-sdk/anthropic exec vitest --config vitest.node.config.js --run src/anthropic-messages-language-model.test.ts
• pnpm --filter @ai-sdk/anthropic exec eslint src/anthropic-messages-language-model.ts src/anthropic-messages-language-model.test.ts
• pnpm --filter @ai-sdk/anthropic type-check
• repeated the same validation suite twice consecutively with zero failures

Closes #12427

Attribution request

If this pull request is merged, I would be grateful if contributor credit could be included in the related changelog or release notes for implementing this fix (@g97iulio1609).