fix: downgrade reasoning parts to text when metadata is stripped

[sudashannon]

When stripStaleMetadata removes provider metadata (including bedrock.signature) from reasoning parts after a model change, the parts were kept as type: "reasoning". Switching back to a thinking-enabled model (e.g., Claude Opus 4-7) causes the AI SDK to create {type: "thinking"} blocks without the required signature field, resulting in API validation errors ("messages.N.content.0.thinking.signature: Field required").

Fix: when metadata is stripped from a reasoning part, downgrade it to type: "text" — the thinking content is preserved as readable text, but the SDK won't construct invalid thinking blocks.

Root cause flow:

1. Fork (or model change) triggers stripStaleMetadata
2. Metadata containing bedrock.signature is stripped from reasoning parts
3. But type: "reasoning" was retained
4. On switching back to a thinking model, the AI SDK converts stored reasoning parts to {type: "thinking"} blocks
5. Without the signature field the API schema validation rejects the request