Support Go 1.27 and expand tool tests

[visualfc]

Summary

• update Go toolchain support and CI matrix to Go 1.25-1.27
• add comprehensive _testdata coverage for types, goapi, astview, docview, and gofmt
• improve generic AST handling and fiximports blank-line normalization

Verification

• PATH="$HOME/golang/go1.27/bin:$PATH" GOROOT="$HOME/golang/go1.27" GOTOOLCHAIN=local go test -count=1 ./...