fix(core): relocate AI SDK tool media - #41843

Summary

• move tool-result images and PDFs into request-local user messages for residual AI SDK providers
• preserve tool-result text and non-image file content while keeping consecutive tool responses together
• keep file-only tool results non-empty after extracting their media

Native provider routes continue to own their protocol-specific media placement. This restores the conservative V1 fallback only for models that still reach the generic AI SDK bridge.

Testing

• bun test test/aisdk.test.ts (from packages/core)
• bun typecheck (from packages/core)
• push hook: full workspace bun turbo typecheck --concurrency=3

Closes #41674