fix(bedrock): tolerate omitted reasoning blocks

[firstprinciples]

Summary

• Accept Bedrock signature-only reasoning blocks produced by thinking.display: "omitted".
• Preserve signed plaintext, signature-only, and redacted reasoning blocks unchanged for replay.
• Reject plaintext reasoning that is missing its provider signature.
• Add non-streaming regression coverage alongside the existing streaming coverage.

Rationale

Anthropic documents thinking.display: "omitted" as returning reasoning blocks with empty text and a signature; this is the default for newer Claude models. Bedrock Converse passes this through.

The adapter previously treated these valid signature-only blocks as malformed. Signature-only reasoning is a complete replay artifact: the signature carries the encrypted reasoning state and must be passed back unchanged for multi-turn continuity. Unsigned plaintext remains unreplayable and is rejected at the provider boundary.

Test plan

• go test ./features/model/bedrock

[raphael]

Reviewed the final diff and provider contract. Signature-only reasoning is preserved for verbatim replay, while unsigned plaintext still fails at the Bedrock boundary. No blocking code findings.

Verified the PR head with:

• go test ./features/model/bedrock

[raphael]

Signature-only reasoning now matches the provider replay contract, unsigned plaintext remains rejected, and the Bedrock package tests pass.