fix(codemod): scope reasoning property migration

[Kage18]

Summary

• scope the v5 reasoning property migration to AI SDK result and step shapes instead of every property named reasoning
• preserve unrelated React props and local objects named reasoning
• add regression fixtures for the reported MessageReasoningProps case and valid AI SDK result/step migrations

Fixes #8340.

Kage usage

• Recalled package context for @ai-sdk/codemod and the reasoning codemod tests/code graph.
• Captured a repo-local memory packet documenting that the false positive came from the broad rename-reasoning-properties step, plus the verified fix and test commands.
• Local Kage metrics for packages/codemod after refresh: 300 files, 1,354 symbols, 1,203 calls, 87 tests, 100% indexed coverage, 3 memory packets, 53 memory graph entities, 54 evidence-backed edges, 0 stale packets.

Tests

• pnpm --filter @ai-sdk/codemod test
• pnpm --filter @ai-sdk/codemod type-check

[Kage18]

Kage usage for this PR:

This PR fixed #8340 by scoping the v5 reasoning-property codemod to AI SDK result/step shapes instead of renaming every property named reasoning.

How Kage helped:

• Recalled the @ai-sdk/codemod package context and the existing reasoning codemod/test structure.
• Kept the implementation and fixture paths together: rename-reasoning-properties.ts, codemod tests, and input/output fixtures.
• Helped separate valid AI SDK result migrations from unrelated React/component props such as MessageReasoningProps.reasoning.
• Captured the durable rule: only rewrite reasoning / reasoningDetails when tied to generateText / streamText results, step values, or explicit AI SDK result annotations.
• Captured the regression evidence and verification commands: pnpm --filter @ai-sdk/codemod test and pnpm --filter @ai-sdk/codemod type-check.

Why this matters for future sessions:
A future agent editing codemods can recall the false-positive class and avoid broad AST rewrites that mutate user-defined props or local objects just because they share an AI SDK field name.

Full contribution summary and Kage metrics: kage-core/Kage#8

[lgrammel]

Thank you for the pull request. Issue #8340 was closed as completed. Closing this pull request.