feat(core): Warn on repeated Sentry.init() and unbind the client on close() - #24962

… `close()`

A repeated `Sentry.init()` call was mostly undefined behavior, and
each SDK handled it in its own way. Most SDKs built a new client and
replaced the old one without a warning. Nothing closed the old client,
so its buffers, timers, and hooks stayed alive, and `setupOnce` kept
the settings of the first call.

The goal is one rule for all SDKs: the first `init()` wins, a later
call returns the active client, and `close()` lets you start over.
Most SDKs cannot switch to "first wins" before a major version, so
this change adds the parts that are safe now:

- `initAndBind`, Node's `_init`, and Vercel Edge's `init` print a
  warning (with or without `debug`) when a client is already bound.
  They still replace the client for now. Wrappers that expect a
  repeated call (Next.js server, Remix server, Nuxt server, Hono) keep
  their own guard and return early, so they do not warn. Cloudflare's
  `cacheClient: false` asks for a new client on each call, so it
  unbinds the old one first and does not warn. The Next.js client drops
  its own warning, which used a flag that never reset and so also fired
  after `close()`. Its config-file hint moves to the docs.
- `Sentry.close()` unbinds the client after it closes it. Before, a
  guard based on `getClient()` treated the closed client as active, so
  `close(); init()` returned the closed client. Nuxt's server guard
  now also checks for a bound client, for the same reason. Cloudflare
  also caches its client for the isolate, and the cache handed the
  closed client back to every later `init()`, so the isolate sent
  nothing until it was recycled. Closing the cached client now clears
  the cache.
- Next.js server and Remix server return the active client from a
  repeated call, not `undefined`. A caller could not tell "already
  initialized" from "failed".

`docs/repeated-init.md` records the rule, the current behavior of
each SDK, and the plan for the next major. The warning text should
point apps that share a page to the isolated client helper from
PR #24883 once that helper has a final name.

The Next.js and Nuxt server `init()` added their event processors to the
global scope. Since `close()` now unbinds the client, a later `init()`
would run the full setup again, and each cycle would add another copy of
each processor to the global scope. So instead, add them to the client,
so that they go away with a closed client, and a later `init()` uses its
own options. Client processors run before all scope processors, so these
now also run before any global scope processor that user code added
before `init()`. They only drop events or fix stack frames, so the
earlier position does not change which events are sent.

Fixes #24960

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>