Fix linting issues: canonicalheader, embeddedstructfieldcheck, iotamixing, makezero, testableexamples, wastedassign, usetesting, tparallel, unconvert, intrange, iface

- canonicalheader: fix cacheTTL header value to canonical form
- embeddedstructfieldcheck: move embedded fields before regular fields in 4 structs
- iotamixing: split const blocks mixing iota with non-iota constants
- makezero: use make([]T, 0, n) instead of make([]T, n) before appending
- testableexamples: add missing Output: comment to ExampleOption_UnwrapOrZero
- wastedassign: remove wasted initial assignments in 4 locations
- usetesting: replace os.MkdirTemp/os.Setenv with t.TempDir/t.Setenv in tests
- tparallel: add t.Parallel() to 8 top-level test functions
- unconvert: remove 16 unnecessary type conversions
- intrange: convert 3 for loops to use integer range syntax
- iface: consolidate identical EditPrompter and Prompt interfaces via type alias

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

internal/update/update_test.go

@@ -416,7 +416,7 @@ func TestShouldCheckForUpdate(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			os.Clearenv()
 			for k, v := range tt.env {
-				os.Setenv(k, v)
+				t.Setenv(k, v)
 			}
 
 			actual := ShouldCheckForUpdate()
@@ -473,7 +473,7 @@ func TestShouldCheckForExtensionUpdate(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			os.Clearenv()
 			for k, v := range tt.env {
-				os.Setenv(k, v)
+				t.Setenv(k, v)
 			}
 
 			actual := ShouldCheckForExtensionUpdate()