#13972)

## background

`trimIfLast()` trims trailing whitespace from reasoning block text even
when a cryptographic `signature` is present. the signature is computed
over the exact original bytes - trimming even a single trailing newline
invalidates the text/signature pair, causing Bedrock to reject with
`ValidationException: thinking blocks cannot be modified`

## summary

- skip `trimIfLast` for reasoning blocks that have a signature
- reasoning blocks without a signature still get trimmed as before
- add test for multi-turn tool use with signed reasoning blocks

## checklist

- [x] tests have been added / updated (for bug fixes / features)
- [ ] documentation has been added / updated (for bug fixes / features)
- [x] a _patch_ changeset for relevant packages has been added (run
`pnpm changeset` in root)
- [x] i have reviewed this pull request (self-review)

## related issues

fixes #13583