Migrate templates to 4.x APIs via @payloadcms/codemod plus manual fixes:
- Move storage adapters from plugins to the top-level storage array (#16596)
- Rewrite removed @payloadcms/next/{rsc,client} subpaths to @payloadcms/ui in import maps
- Replace bare src/ imports with the @/ path alias
- Drop the slug generic on Pages/Posts so defaultPopulate stays assignable to the collections array

# Overview

The `build-and-test-templates` CI job was disabled while 4.x breaking
changes landed, so the templates drifted and no longer build against
current core packages. This updates each template to the new 4.x APIs so
they build again.

CI for the re-enabled job:
[#16880](#16880).

## Key Changes

- **Storage adapters moved from `plugins` to `storage`**
- 4.x initializes storage adapters from a top-level `storage` array
instead of `plugins` (#16596). `vercelBlobStorage` / `r2Storage` calls
were relocated in `with-vercel-mongodb`, `with-vercel-postgres`,
`with-vercel-website`, and `with-cloudflare-d1`. This was the source of
the `TypeError: plugin is not a function` failures. Applied with
`@payloadcms/codemod`'s `migrate-storage-adapters-to-config`.

- **Import maps point at `@payloadcms/ui` instead of removed
`@payloadcms/next` subpaths**
- The `@payloadcms/next/rsc` and `@payloadcms/next/client` subpaths were
removed; generated import maps now reference `@payloadcms/ui/rsc` and
`@payloadcms/ui`. Applied with `@payloadcms/codemod`'s
`migrate-next-subpath-exports` across all templates.

- **Bare `src/` imports replaced with the `@/` path alias**
- Website-based templates imported from `src/...`, which no longer
resolves under Turbopack. Switched to the configured `@/` alias (maps to
`./src/*`).

- **Slug generic dropped on `Pages` / `Posts` (TypeScript 6 regression
workaround)**
- The documented `CollectionConfig<'pages'>` + `defaultPopulate` pattern
compiled under TypeScript 5.7 (3.x) but not under TypeScript 6.0.3 (the
#16692 bump): the generated select interface is no longer assignable to
the `SelectType` that the `collections` array element expects. Dropping
the generic lets `defaultPopulate` fall back to `SelectType` so the
templates build, at the cost of field-name checking on
`defaultPopulate`. This is a core type regression that affects any 4.x
project using the pattern under TS 6, so the real fix belongs in core;
the templates carry a commented workaround to be reverted once that
lands.

## Design Decisions

Most changes were applied with `@payloadcms/codemod` rather than by
hand, so the same transforms users will run produce the same result.
Import maps are left as the minimal subpath rewrite rather than a full
local regeneration, since `payload build` regenerates them during the
build.

Scope is template builds. All six matrix templates build with packed
local packages, verified locally via `script:pack --all` +
`build-template-with-local-pkgs`.

The remaining E2E failures on `blank` and `website`
(`chunk.reason.enqueueModel is not a function` when loading the admin
under `next dev`) are a pre-existing runtime issue, present before this
change and unrelated to the build fixes. Only the workspace templates
(`blank`, `website`, `ecommerce`) run E2E in this job; `pnpm --filter`
does not match the non-workspace templates, so their E2E and int steps
currently no-op.


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- To see the specific tasks where the Asana app for GitHub is being
used, see below:
  - https://app.asana.com/0/0/1215426886664378