test(agentkind): expand Codex backend coverage

[winterfx]

Closes #3

Summary

• Added deterministic fake app-server scenarios for retriable and terminal Codex errors.
• Verified retriable errors allow the turn to recover while terminal errors emit error and failed done events.
• Added assertions for file-change tool events, affected locations, tool results, and reasoning events.
• Isolated fake-server tests from host Codex configuration by assigning a temporary CODEX_HOME.
• Increased package statement coverage to 78.4%.

Validation

• cd backend-go && go test ./pkg/agentkind -count=1 — passed: All agentkind tests passed.
• cd backend-go && go test ./pkg/agentkind -count=1 -coverprofile=/tmp/agentkind-cover-final.out — passed: All tests passed with 78.4% statement coverage.
• git diff --check — passed: No whitespace errors found.

Risk

• Level: low
• Only backend-go/pkg/agentkind/codex_test.go changed.
• Changes add deterministic test scenarios and test-environment isolation without modifying production behavior.

Notes

• No commits, pushes, provider API calls, or credential access were performed.
• The requested workspace was the only repository workspace accessed.

Prepared by the repository development Agent and opened as a Draft Pull Request for maintainer review.