Issue for this PR

V2 follow-up to #45621. This changes the native Bedrock Converse adapter, not V1 or the Vercel SDK dependency.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Unsigned reasoning can reach the Bedrock adapter after a model switch or from incomplete history without a recorded error. The adapter currently sends it as reasoningContent without a usable signature.

Demote readable unsigned reasoning to ordinary text, preserving its cache hint, as the Anthropic adapter already does. Drop blank unsigned reasoning and omit assistant messages left empty. Signed and redacted reasoning remain unchanged, without cache points appended to those blocks.

How did you verify your code works?

• Eight new request-compilation regressions failed before the fix and pass afterward: missing/empty/whitespace signatures with automatic caching on/off, empty reasoning with cache hints, and mixed foreign/signed/redacted reasoning plus text and tools.
• Bedrock Converse, Bedrock cache replay, and Anthropic Messages suites: 122 passed.
• Core history conversion and interrupted-reasoning continuation checks: 24 passed.
• AI package typecheck, formatting, and git diff --check passed.
• Full AI suite: 789 passed, 25 skipped, 17 failed. Untouched base 5d4ce14dc4 has the identical 17 failing test names (781 passed, 25 skipped); those existing failures are not changed here.
• No live Bedrock request was made; validation uses local wire-shape assertions and existing fixtures.

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

Requested by: @neriousy (Filip via Slack)