fix(opencode): request summaries in Bedrock GPT-5 variants

[rekram1-node]

Summary

• Add additionalModelRequestFields.reasoning.summary: "auto" to Bedrock Converse GPT-5 variants, in both catalog-driven and fallback variant generation.
• Mantle already sets reasoningSummary: "auto"; leave it unchanged.
• Leave effort mapping, defaults, configuration merging, and request preparation unchanged.

This replaces the earlier, broader implementation. The final diff contains only the variant summary setting and two focused regression tests.

Verification

• Both added tests failed without the summary setting and pass with it.
• Provider transform and Bedrock suites: 448 passed, 0 failed.
• Package typecheck, Prettier check, and git diff --check passed.

This requests summaries; it does not guarantee AWS returns readable summary text or address the separately identified effort-serialization issues.