test(go): expand helper coverage

[halukerenozlu]

What does this PR do?

Expands low-risk helper test coverage for user-friendly error handling and session UI helper functions. This is a test-only change and does not modify runtime behavior.

Related issues

N/A

Changes

• Add tests for not-found error formatting, path extraction, suggestions, and redaction
• Add tests for typed command normalization and approval response labels
• Add tests for session model/provider helper functions
• Add tests for session formatting, truncation, token estimate, and pricing helpers

How to test

Steps a reviewer can follow to verify the change manually:

1. Run go test ./... -count=1
2. Run go test -coverprofile .\coverage.out ./...
3. Run go tool cover -func .\coverage.out and confirm total coverage is around 70.1%

Checklist

• Tests pass (go test ./... -count=1)
• No vet warnings (go vet ./...)
• Build succeeds (make build)
• Conventional commit message (e.g. feat(go/agent): ...)
• Docs updated if behaviour changed
• No breaking changes (or described below)