chore: update golangci-lint to v2.12.2 with canonical config

Replace .golangci.yml with the canonical v2-schema config
(default: all minus six disabled linters, lll 88, tests included)
and bump every golangci-lint pin to v2.12.2:

- Dockerfile: golangci/golangci-lint:v2.12.2-alpine (hash-pinned)
- script/bootstrap: GOLANGCI_LINT_VERSION 2.12.2 with new
  linux-amd64/arm64 release-archive sha256 pins

Fix all 747 findings the stricter config surfaces, with no behavior
changes: t.Parallel() throughout the test suite, static sentinel
errors and errors.Is comparisons, checked error returns, context
propagation (contextcheck/noctx), 88-column wrapping, extracted
constants and helpers for goconst/dupl/funlen/cyclop, exhaustive
switch cases replicating existing defaults, and white-box test files
renamed to *_internal_test.go for testpackage. Three
nolint:tagliatelle directives preserve the existing snake_case JSON
wire and on-disk metadata formats.

internal/session/session.go

@@ -107,7 +107,9 @@ func (m *Manager) ValidateSession(r *http.Request) (*Data, error) {
 	}
 
 	var data Data
-	if err := m.sc.Decode(CookieName, cookie.Value, &data); err != nil {
+
+	err = m.sc.Decode(CookieName, cookie.Value, &data)
+	if err != nil {
 		return nil, ErrInvalidSession
 	}