test: expand Go fuzzing and raise coverage above 80%

- Add comprehensive runtime tests for session, docker watcher/stats/http, and server helpers
- Add new fuzz targets: FuzzToIntFromQuery and FuzzHTMLHelpers
- Add cmd/webterm main entrypoint test
- Expand helper conversion and color-mode coverage tests

Validation:
- go test ./...
- go test -race ./...
- All 15 fuzz targets pass with short fuzz runs
- go test ./... -coverpkg=./... => 80.9% total statement coverage

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

go/cmd/webterm/main_test.go

@@ -0,0 +1,13 @@
+package main
+
+import (
+	"os"
+	"testing"
+)
+
+func TestMainVersionFlag(t *testing.T) {
+	orig := os.Args
+	defer func() { os.Args = orig }()
+	os.Args = []string{"webterm", "-v"}
+	main()
+}