Refactor networking from signalling into a new livekit-net crate (#1258)

Supersedes #1229.
Fixes CLT-2177

This PR is the first of three:

- [ ] refactor. This moves networking from signalling into a new
livekit-net crate.
- [ ] expand. This adds support for the livekit-net transport to be
injected by the host platform
- [ ] migrate
- [ ] contract. This removes support for the default native transport
provided by reqwest and tungstenite.

`livekit-net` introduces a `PlatformTransport` trait, which client
crates then use to call out to the network. Start in the
`livekit-net/src/transport.rs` file.

This covers the websocket interface and http, so feasibly we might have
also call this network client.

The large line difference:

- about +762-804 is just for `Cargo.lock`
- almost all of the new livekit-net crate is just cut/paste/adapt from
livekit-api: +946 -0 for the livekit-net, then +214 −962 for the removal
commit.

### Before you submit your PR

Make sure the following is true before submitting your PR:

- [ ] I have read the [contributing
guidelines](https://github.com/livekit/rust-sdks/blob/main/CONTRIBUTING.md)
and validated that this PR will be accepted.
- [ ] I have read and followed the principles regarding breaking
changes, testing, and code quality.

### PR description

Describe the changes in this PR. Explain what the PR is meant to solve
and how to reproduce the issue in the first place.

### Breaking changes

If this PR introduces breaking changes, list them here and document the
rationale for introducing such a change.

### MSRV

If the PR modifies the crate's MSRV (Minimum Supported Rust Version),
document it here.

### Testing

Ideally, unit test the code you add, but ensure you're not repeating
existing test cases. Use as many already written scaffolding, utilities
as possible; write your own, when needed. If external services, APIs,
tokens are required (e.g., running an LK server instance), provide the
necessary information. Make sure your tests perform useful,
context-aware assertions and do not simply emulate "happy paths".

### Async

We want the project to be runtime-agnostic, so please reuse what's
already in
[livekit-runtime](https://github.com/livekit/rust-sdks/blob/main/livekit-runtime/)
and feel free to add anything missing. It's ok to use Tokio directly,
when writing unit tests, if necessary. When testing, do not use
artificial delays for the state to "catch up"; instead, respect the
event flow and subscribe properly using channels or other mechanisms.

Author: jhugman

.changeset/livekit-net-transport-seam.md

@@ -0,0 +1,17 @@
+---
+livekit-net: minor
+livekit-api: major
+livekit: major
+livekit-ffi: patch
+livekit-uniffi: patch
+---
+
+Route LiveKit signalling through a pluggable transport (new `livekit-net` crate).
+
+The signalling WebSocket and the two pre-connect HTTP GETs (validate, region discovery) now go through pluggable transport traits (`WsClient` for the WebSocket, `HttpClient` for request/response) resolved from a process-global registry with independent slots — a consumer can bring only HTTP, or only WebSocket. The new `livekit-net` crate owns the WebSocket/HTTP/TLS stack behind those traits and ships native (tokio / async-std) backends. Native builds are unchanged in behavior.
+
+**Breaking (`livekit-api`, and `livekit` via `EngineError::Signal`):**
+
+- `SignalError::WsError` is removed — `tungstenite` is no longer part of the public API. A failed WebSocket handshake now surfaces its HTTP status as `SignalError::Client`/`Server`; transport connection and close failures surface as the new `SignalError::Connection(String)` / `SignalError::Closed` variants (previously all collapsed into `Timeout`).
+- `SignalError` is now `#[non_exhaustive]`, and gains a `SignalError::TransportNotConfigured` variant — returned when no transport is registered (host/foreign builds must call `livekit_net::set_ws_client` / `set_http_client` before connecting). This is a permanent configuration error; callers must not retry.
+- The signalling WebSocket/HTTP/TLS crates are no longer transitive dependencies of `livekit-api`; TLS features delegate to `livekit-net`. Existing `signal-client-tokio` / `-async` / `-dispatcher` and TLS feature names are unchanged.

.github/workflows/tests.yml

@@ -162,4 +162,10 @@ jobs:
             --features __lk-e2e-test \
             -- \
             --nocapture \
-            --test-threads=1
+            --test-threads=1
+
+      - name: Test transport seam (livekit-net / signal-client)
+        shell: bash
+        run: |
+          cargo test --verbose --target ${{ matrix.target }} -p livekit-net --features native-tokio
+          cargo test --verbose --target ${{ matrix.target }} -p livekit-api --features signal-client-tokio