Fix custom server conf propagation

[KurodaKayn]

Fixes #87429

What?

Ensures config passed through next({ conf }) in a custom server is applied consistently across the router and render server pipeline.

Why?

Custom server config such as basePath was loaded by the router server but not propagated into the internal render server. As a result, requests using basePath could still render with the default config and return 404.

How?

• Pass conf from NextCustomServer.prepare() to getRequestHandlers()
• Forward conf through router-server.initialize() into loadConfig()
• Forward conf into renderServerOpts so the internal render server uses the same config
• Preserve the custom server marker when initializing request handlers
• Add an integration test covering basePath passed through next({ conf })

Verification

• pnpm install --frozen-lockfile
• pnpm --filter=next build
• NEXT_SKIP_ISOLATE=1 NEXT_TEST_MODE=dev pnpm testheadless test/integration/custom-server/test/index.test.ts -t "with config provided through next"
• git diff --check