fix(helm): use documented image values instead of undocumented _image key (#41765)

## Summary

- Fixes #40461 — custom `image.registry`, `image.repository`, and
`image.tag` values in `values.yaml` were silently ignored because the
deployment template read from `.Values._image` (undocumented, never set)
instead of `.Values.image`
- Documented the deprecated _image values key (currently used by
consumers) in `deploy/helm/values.yaml` and
`deploy/helm/values.schema.json`, and clarified that it merges on top of
image (higher precedence).
- Fixed an inconsistency in `deploy/helm/templates/deployment.yaml` so
imagePullSecrets is sourced from the merged effective image config (same
source as tag/repo/pullPolicy), ensuring `_image.pullSecrets` behaves
consistently during the deprecation window.
- Corrected/clarified the image.tag default wording to match templating
behavior (defaults to "latest" when omitted, not “chart appVersion”).

## Backward compatibility

Existing installs using `_image` continue to work. `_image` remains an
override layer on top of image and is now explicitly documented as
deprecated.

## Test plan

- [x] `helm unittest deploy/helm` — 53 tests passing (includes
`tests/image_test.yaml`)
- [x] `helm template --set image.registry=myregistry.io` — verified
custom registry rendered correctly
- [x] `helm template --set _image.registry=myregistry.io` — verified
backward compat still works
- [x] Live `kind` cluster install with `--set
image.registry=myregistry.io` — pod spec confirmed
`myregistry.io/appsmith/appsmith-ee:latest`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: Cypress test results  -->
> [!WARNING]
> Tests have not run on the HEAD
0fdc11dc6650a78187879aefaa58ffbb2994833d yet
> <hr>Wed, 29 Apr 2026 20:13:21 UTC
<!-- end of auto-generated comment: Cypress test results  -->


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Tests**
* Added comprehensive Helm tests validating container image rendering
across scenarios (registry, repository, tag overrides, and precedence),
including imagePullPolicy behavior.

* **Chores**
* Improved deployment image resolution and override precedence so
partial or full image overrides behave predictably and pull policy
follows explicit overrides.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Wyatt Walter <wyattwalter@gmail.com>