feat: add --repo-retries for helm repo and registry login commands (#2683)

* feat: add --repo-retries for retrying helm repo and registry login commands

Add a configurable retry mechanism for chart repository operations to
handle unstable networks (corporate proxies, slow internal registries).

Closes #1894

- New --repo-retries N flag and HELMFILE_REPO_RETRIES env var (default 0
  = opt-in, backward compatible)
- Retry applies to helm repo add, helm repo update (incl. ACR), and
  helm registry login with exponential backoff (1s, 2s, 4s, ..., capped 30s)
- Single retryRepoOp helper; per-attempt args/buffer are local to avoid
  state leaking across retries
- Tests cover succeed-after-retry, exhausted-retries, disabled-by-default,
  and regression guards for password-buffer and args-accumulation

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

* fix: address PR review (overflow guard, cancellable sleep, flag-override, docs)

Address Copilot review feedback on #2683:

- Cap backoff shift exponent at 5 to prevent time.Duration overflow on
  large --repo-retries values
- Make retry sleep context-aware (sleepCtx) so Ctrl+C aborts the retry
  loop promptly via the ShellRunner context
- Log a concise exit status instead of the verbose ExitError dump, and
  clarify the retry-counter wording ('retry N/M')
- Use -1 sentinel as the CLI default so --repo-retries=0 can explicitly
  disable retries even when HELMFILE_REPO_RETRIES is set
- Align help text and docs: retry applies 'on failure' (not just
  transient errors), document the 0-disables behavior
- Add tests for overflow guard, cancellable sleep, and flag-zero-disables

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

* fix: abort retries on canceled context, hide sentinel default, align comment

Address follow-up Copilot review on #2683:

- Fix tight-loop bug: sleepCtx now returns whether it completed vs was
  interrupted by context cancellation, and retryRepoOp aborts the retry
  loop on interruption so Ctrl+C no longer spins into rapid helm calls
- Hide the -1 sentinel from --help by overriding the displayed default
  to 0 (pflag DefValue), matching the documented default while keeping
  the flag-override semantics
- Correct HelmExecOptions.RepoRetry comment: 'on failure' not 'transient
  network errors', matching the actual retry behavior
- Add Test_Retry_AbortsOnCanceledContext covering the no-tight-loop path

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

* fix: copy args per retry in RegistryLogin, make cancel test deterministic

Address follow-up Copilot review on #2683:

- RegistryLogin: pass a per-attempt copy of args to execStdIn so its
  internal append (for helm.extra) can't alias the shared slice across
  retries
- Test_Retry_AbortsOnCanceledContext: cancel the context deterministically
  inside the op closure after the first attempt, replacing the flaky
  time.Sleep(20ms) goroutine

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

* fix: return error on unknown managed repo type instead of silent skip

Address Copilot review on #2683: AddRepo logged an error for an unknown
managed type but returned nil, silently succeeding while skipping the
repo add. Now returns an error so misconfigurations fail loudly.

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

---------

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

docs/cli.md

@@ -55,6 +55,7 @@ Flags:
   -n, --namespace string                      Set namespace. Overrides "HELMFILE_NAMESPACE" OS environment variable when specified. Uses the namespace set in the context by default, and is available in templates as {{ .Namespace }}
       --no-color                              Output without color. Overrides "HELMFILE_NO_COLOR" and "NO_COLOR" OS environment variables when specified
   -q, --quiet                                 Silence output. Equivalent to log-level warn. Overrides "HELMFILE_QUIET" OS environment variable when specified
+      --repo-retries int                      Number of times to retry "helm repo add/update" and "helm registry login" on failure, with exponential backoff (1s, 2s, 4s, ..., capped at 30s). Set to 0 to disable retries. Overrides "HELMFILE_REPO_RETRIES" OS environment variable when specified
   -l, --selector stringArray                  Only run using the releases that match labels. Labels can take the form of foo=bar or foo!=bar.
                                               A release must match all labels in a group in order to be used. Multiple groups can be specified at once.
                                               "--selector tier=frontend,tier!=proxy --selector tier=backend" will match all frontend, non-proxy releases AND all backend releases.
@@ -547,6 +548,7 @@ The following global flags are also available but not shown in the main help out
 | `--skip-refresh` | false | Skip running `helm repo update` (lighter than `--skip-deps` which also skips dependency build) |
 | `--enforce-plugin-verification` | false | Fail plugin installation if verification is not supported |
 | `--oci-plain-http` | false | Use plain HTTP for OCI registries (required for local/insecure registries in Helm 4) |
+| `--repo-retries` | `0` | Number of times to retry `helm repo add/update` and `helm registry login` on failure, with exponential backoff (1s, 2s, 4s, ..., capped at 30s). Set to 0 to disable retries. Overrides `HELMFILE_REPO_RETRIES` |
 
 #### fetch flags