Add warning when zen.tool.go does not exist in main package

[tomaisthorpe]

zen.tool.go must be in the same folder as the main package, if it's not there, it can create linker issues which are not obvious. Added a warning to help diagnose the problem.

Example:

❯ go build -toolexec="zen-go toolexec" -o app ./cmd
# myapp/cmd
zen-go: warning: zen.tool.go not found in cmd. zen.tool.go must be in the same directory as your main package. Run 'zen-go init' from that directory to set up instrumentation.
# myapp/cmd
cannot find package github.com/AikidoSec/firewall-go/instrumentation/sources/labstack/echo.v4 (using -importcfg)
/opt/homebrew/Cellar/go/1.26.1/libexec/pkg/tool/darwin_arm64/link: cannot open file : open : no such file or directory
exit status 2

Summary by Aikido

Security Issues: 0	🔍 Quality Issues: 1	Resolved Issues: 0

⚡ Enhancements

• Emitted warning when zen.tool.go missing from main package build.
• Modified checkZenToolFileIncluded to accept stderr and print warnings.

^{More info}