test: add integration test for selector filtering with requiredEnv (#1172) (#2678)

* fix: retry rendering with lenient requiredEnv when selectors are active (#1172)

The entire helmfile document is rendered as a Go template before selector
labels filter releases. This means requiredEnv calls in releases excluded
by selectors still fail, blocking the whole run.

When selectors are active and rendering fails due to a requiredEnv error,
helmfile now retries with lenient mode: unset env vars produce empty
strings instead of failing. The document can then be parsed and filtered
by selectors normally.

Behavior:
- Without selectors: requiredEnv fails as before (validation preserved)
- With selectors, all env vars set: strict render succeeds, no retry
- With selectors, some env vars missing: lenient retry, excluded releases
  get empty values and are filtered out by selectors

Implementation:
- Add RequiredEnvError type + ErrRequiredEnvNotSet sentinel for type-safe
  error detection via errors.As
- Add lenientRequiredEnv flag to tmpl.Context; requiredEnv returns empty
  string instead of failing when set
- Add NewLenientFileRenderer via functional options pattern
  (FileRendererOption / WithPreRender / WithLenientRequiredEnv)
- Extract renderWithSelectorFallback in two_pass_renderer.go
- Pass selectors from LoadOpts to desiredStateLoader

Closes #1172

Signed-off-by: yxxhero <yxxhero@example.com>
Signed-off-by: yxxhero <aiopsclub@163.com>

* test: add integration test for selector filtering with requiredEnv (#1172)

Verify that selector-based filtering works correctly when requiredEnv is
used in release values. Users set all required env vars, so rendering
succeeds, and selectors filter out non-matching releases.

Test scenarios:
- helmfile template -l tier=label2: only rel2 is templated (rel1 excluded)
- helmfile template without selector: both releases are templated

Closes #1172

Signed-off-by: yxxhero <yxxhero@example.com>
Signed-off-by: yxxhero <aiopsclub@163.com>

---------

Signed-off-by: yxxhero <yxxhero@example.com>
Signed-off-by: yxxhero <aiopsclub@163.com>

test/integration/run.sh

@@ -147,6 +147,7 @@ ${kubectl} create namespace ${test_ns} || fail "Could not create namespace ${tes
 . ${dir}/test-cases/issue-1880-transformers-with-file-deps.sh
 . ${dir}/test-cases/issue-821-adhoc-dep-go-getter.sh
 . ${dir}/test-cases/issue-2599-default-inherit.sh
+. ${dir}/test-cases/issue-1172-selector-required-env.sh
 . ${dir}/test-cases/kubedog-tracking.sh
 . ${dir}/test-cases/lookup.sh