fix(ai): round-trip Bedrock redacted reasoning (#8314)

Bedrock Converse returns encrypted reasoning from non-Anthropic models
(e.g. OpenAI GPT-5.6) as the opaque `reasoningContent.redactedContent`
member. The stream handler only read `text` and `signature`, so the
payload was dropped and replayed history lost the reasoning block.

Buffer the delta bytes, encode them into `thinkingSignature` with
`redacted: true` (the representation the Anthropic path already uses),
and lower them back to `reasoningContent.redactedContent` on the next
turn. Scratch state is stripped from every terminal path, since a
stream can settle without stopping each block.