fix(cache): skip env-requiring cache adapters when no env is supplied

[Divkix]

Fixes #3132

Summary

• Add a requiresEnv flag to CacheAdapterDescriptor. The generated virtual:vinext-cache-adapters registrar emits that slot's registration inside if (env != null) { ... }, so a runtime that supplies no env skips it silently instead of invoking a factory that cannot construct and warning about it.
• kvDataAdapter() sets requiresEnv: true, so Node runs (vinext build prerender, vinext start, Node dev) keep the default in-memory handler with no warning about a VINEXT_KV_CACHE binding that cannot exist off the Workers runtime.
• Behaviour is unchanged where it matters: Workers callers always pass env, so registration happens exactly as before; an env-present caller whose binding is genuinely missing still logs the existing warning; adapters that never opt in keep the previous attempt-and-warn behaviour.
• One docs line in docs/caching.md.

Test plan

• tests/cache-adapters-config.test.ts: guarded-codegen shape, descriptor flag, and executed-registrar behaviour (skip without env / register with env / warn when the binding is genuinely missing / non-opted adapters unchanged).
• vp test run --project unit tests/cache-adapters-config.test.ts tests/cache-adapters-build.test.ts tests/image-adapters-config.test.ts — 65 passed.
• End-to-end repro of the issue with App Router and Pages Router apps (cache: { data: kvDataAdapter() }, prerender: { routes: '*' }): vinext build and vinext start emit no warning and serve 200.
• Workers path: the built Cloudflare worker bundle keeps the guarded registration and still runs under workerd with the KV binding present.
• CI-equivalent local runs: check, knip, unit, integration, package tests, the full Playwright project matrix, create-next-app (node and cloudflare init) and create-vinext-app (Cloudflare build). The only failures are pre-existing on a pristine e97de291 baseline in this macOS environment (path-derived action ids, temp-path fixtures).

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commit: 6921152

[github-actions]

Performance benchmarks

Compared 6921152 against base e97de29 using alternating same-runner rounds. Next.js was unchanged and skipped.

0 improved · 0 regressed · 6 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	142.2 KB	142.2 KB	⚫ -0.0%
Client entry size (gzip)	vinext	129.6 KB	129.6 KB	⚫ -0.0%
Dev server cold start	vinext	3.17 s	3.12 s	⚫ -1.4%
Production build time	vinext	3.61 s	3.61 s	⚫ -0.2%
RSC entry closure size (gzip)	vinext	121.0 KB	121.0 KB	⚫ -0.0%
Server bundle size (gzip)	vinext	218.3 KB	218.3 KB	⚫ -0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}