test(go): improve coverage baseline

[halukerenozlu]

What does this PR do?

Improves the coverage baseline with low-risk test-only changes. It excludes MCP test helper packages from Codecov reporting and adds behavior-focused tests for builtin file tools plus lightweight UI panel/widget tests.

Related issues

N/A

Changes

• Add codecov.yml to ignore internal/mcp/testutil/** from Codecov coverage
• Add tests for builtin tool metadata, schemas, required input validation, and success paths
• Add basic Bubble Tea contract tests for UI panels
• Add plan, spinner, and approval widget tests

How to test

Steps a reviewer can follow to verify the change manually:

1. Run go test ./...
2. Run go test -coverprofile .\coverage.out ./...
3. Run go tool cover -func .\coverage.out and confirm total coverage is around 65.2%

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)