Remove more as any casts with behaviour-preserving changes

[jamiboym]

Stacked on #100418 (type-only changes). Review that one first; this PR's diff is only the second commit.

Summary

Removes another ~156 as any casts: 62 in packages/next/src (153 → 91) and ~94 in tests. Unlike the previous PR, some of these change runtime code. Each one still behaves identically for every input, including null/primitives, Proxies, setters, frozen objects, this binding and evaluation order. Rewrites that only differ in edge cases (in checks, new null guards, ?., coercions) are deliberately left out.

Tests

• Read isNextDev / isNextDeploy / isNextStart from e2e-utils instead of (global as any). nextTestSetup() returns getters for the same exports.
• The global is true | undefined and the export is a boolean. Every use is in a boolean context (if, !, ternaries, defaulted params), so the two are interchangeable.
• next-test-utils keeps its global read: it is also loaded without e2e-utils, and importing that module would run its side effects.

packages/next

• Object.assign(target, { k: v }) instead of a cast plus property assignment, with the same [[Set]], the same order and the same strict-mode throw behaviour. This covers:
  • babel parse errors
  • the prerender-interrupted and bubbled base-server errors
  • _ogBody
  • the resolved IO promise
  • the redirect error's handled flag
  • the telemetry response
  • NEXT_BUILTIN_DOCUMENT
  • the sandbox router context
• Small typed helpers with identical calls: inLayerGroup, excludesFalse, and a type-predicate arrow for .filter(Boolean).
• Typed .call/.apply for the process and crypto patches in node-environment-extensions; this and arguments pass through unchanged.
• Removes the unused future plumbing from the webpack config context; nothing reads ctx.future.
• A few type-only leftovers:
  • overloads for resolveAsArrayOrUndefined
  • an internal __appRouter marker on CompletePrivateRouteInfo
  • a nullable dev-only WeakMap in use-action-queue
  • dropped casts in headers.test.ts / request-cookies.test.ts, now that the sealed types declare those methods

Verification

• Wrote an equivalence argument for each runtime change. The commit message summarises them.
• tsc --noEmit for packages/next and for the root project (tests). The root check's only errors are next/dist resolution misses in untouched tests, because the local worktree has no build.
• Colocated jest suites for touched files, plus test/unit incremental-cache / sandbox / webpack-config suites. All pass except the 2 fast-set-immediate.external.test.ts tests that also fail on canary locally.
• Not run: e2e suites touched by the isNextX swap (e.g. app-static, middleware-general, prerender), which need a full build; relying on CI.

[github-actions]

Failing test suites

Commit: 6dea1a6 | About building and testing Next.js

pnpm test-deploy test/e2e/app-dir/app-static/app-static.test.ts (job)

• app-dir static/dynamic handling > should cache correctly handle JSON body (DD)

Expand output

● app-dir static/dynamic handling › should cache correctly handle JSON body

thrown: "Exceeded timeout of 60000 ms for a test.
Add a timeout value to this test to increase the timeout, if this is a long-running test. See https://jestjs.io/docs/api#testname-fn-timeout."

  571 |           gatedBodies.has(callback)
  572 |         ) {
> 573 |           return Reflect.apply(target, thisArg, args)
      |                          ^
  574 |         }
  575 |
  576 |         const applicable = [...describeGateStack]

  at Object.apply (lib/gate/runtime.ts:573:26)
  at Object.apply (lib/e2e-utils/index.ts:66:30)
  at it (e2e/app-dir/app-static/app-static.test.ts:4459:3)
  at Object.describe (e2e/app-dir/app-static/app-static.test.ts:18:1)