Fix Go review reliability and provider handling

[AbirAbbas]

Summary

Fix four production bugs in the Go port that could launch the wrong harness, reject coverage reviews, publish false green reviews, or lose review results during transient GitHub failures.

Changes

• Fix provider-aware harness binary selection, with optional PR_AF_HARNESS_BIN override. This prevents Codex from accidentally executing the Opencode binary and crashing.
• Make all registered reasoner array schemas OpenAI strict-mode valid by declaring items types. This prevents coverage-gate requests from failing with invalid_json_schema.
• Propagate review-dimension failures so an all-dimensions crash fails without posting to GitHub, while partial failures proceed with degraded-dimension metadata. This prevents false green Looks Good reviews.
• Retry review creation up to three times for transient 5xx/network failures with exponential backoff, without retrying 4xx errors. This prevents computed reviews from being dropped after GitHub timeouts.

Test plan

• Run gofmt across the Go module.
• Run go test ./... in go/.
• Added unit coverage for provider mapping, strict schema arrays, all/partial/zero dimension failures, and 5xx/4xx review-post behavior.