SwarmBuilder::with_websocket hardcodes dns::tokio::Transport::system, bypassing with_dns_config (breaks on Android)

Nobody has claimed this yet.

## Summary

The tokio flavour of `SwarmBuilder::with_websocket` unconditionally wraps its inner TCP transport in a **system-config** DNS transport:

https://github.com/libp2p/rust-libp2p/blob/93c505994dfb99bf47f44a415d65d4eefa66ab02/libp2p/src/builder/phase/websocket.rs#L118-L127

This ignores any resolver configuration supplied earlier via `with_dns_config`. On targets where the system resolver is unavailable, the websocket phase fails even though the user explicitly configured a custom resolver one phase earlier.

**Concrete case (Android):** hickory-resolver 0.26's `system_conf` reads the system DNS via JNI (`ndk-context`). In host environments that never initialize the ndk context — e.g. React Native / uniffi mobile apps, where no `android-activity`-style glue runs — `read_system_conf()` fails with `android context was not initialized`. Switching the DNS phase to `with_dns_config(custom_cfg, opts)` fixes that call site, but `with_websocket` then constructs a *second*, system-config resolver and fails with the byte-for-byte identical error. That makes it genuinely confusing to debug: it looks exactly like the bug you just fixed.

## Expected behavior

One of:
- `with_websocket` reuses the resolver configuration established in the DNS phase (or skips DNS-wrapping entirely when the outer transport is already DNS-wrapped);
- a variant like `with_websocket_dns_config(cfg, opts, ...)` exists so the resolver can be supplied;
- at minimum, the `with_websocket` docs mention the hidden `Transport::system` call and its platform requirements.

## Actual behavior

`with_websocket` always calls `libp2p_dns::tokio::Transport::system` and returns `WebsocketError(Dns(...))` on Android RN hosts, regardless of `with_dns_config`.

## Workaround

On Android we skip the websocket transport via the `WebsocketPhase::with_relay_client` shortcut (which calls `without_websocket()` internally). Acceptable when websocket has no consumers on that platform, but it forks the builder chain per target and silently removes `/ws`/`/wss` dial capability.

## Version

- master @ 93c5059 (`libp2p/src/builder/phase/websocket.rs:123`)
- also present in libp2p 0.56.0 (same line in the `impl_websocket_builder!` expansion)
- hickory-resolver 0.26.1

## Would you like to work on fixing this bug?

Happy to send a PR once maintainers agree on the preferred direction.

Start with libp2p/src/builder/phase/websocket.rs around lines 118-127 and trace how the websocket phase constructs its DNS transport after with_dns_config. Compare the available expected approaches and verify the behavior on Android when a custom resolver is configured. Done means websocket setup no longer unconditionally uses the system resolver or breaks the configured DNS path.

Written by the indexing model from the issue text.