fix(tests): break the irtt-client/irtt-server dev-dependency cycle

irtt-client and irtt-server mutually dev-depended on each other for
cross-crate integration tests. That cycle blocks `cargo publish` for
either crate whenever the other doesn't already exist on the registry
at the required version, which is exactly the situation for a crate's
first-ever release (irtt-server has never been published before).

Introduce a new `irtt-integration-tests` crate (publish = false, no
library target) to own tests whose actual subject is the interaction
between two or more irtt-* production crates:

  crate-local behavior          -> crate's own tests/
  cross-crate/system behavior   -> irtt-integration-tests

Move every test that imports both irtt_client and irtt_server there:
the client-side support harness (hmac, large_packets, protocol_options,
fill_negotiation) and the server-side real-socket tests that drive a
real AsyncClient against a real Server (async_client, negotiation_policy,
fill_wire_bytes, traffic_class, wildcard_source). server_set.rs stays in
irtt-server/tests since it only uses irtt_proto as test machinery to
drive irtt-server directly, which is a normal one-directional test
dependency, not a peer relationship.

irtt-server's dev-dependency on irtt-client is dropped entirely now
that nothing under crates/irtt-server/tests needs it, which fully
breaks the cycle from that side.

irtt-client keeps a one-directional dev-dependency on irtt-server: a
handful of white-box unit tests in src/async_client/tests.rs assert on
private AsyncClient internals (socket, remote, test_hooks) while also
needing a real in-tree server for blocking/async equivalence checks, so
they can't move to an external crate without losing that access. Their
InTreeServer helper is duplicated locally rather than shared, matching
the existing precedent of irtt-cli's own tests/support/in_tree_server.rs.

Cargo.lock

@@ -719,6 +719,16 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "irtt-integration-tests"
+version = "0.0.0"
+dependencies = [
+ "irtt-client",
+ "irtt-proto",
+ "irtt-server",
+ "tokio",
+]
+
 [[package]]
 name = "irtt-proto"
 version = "0.5.0"
@@ -734,7 +744,6 @@ name = "irtt-server"
 version = "0.5.0"
 dependencies = [
  "getrandom 0.4.3",
- "irtt-client",
  "irtt-proto",
  "nix 0.31.3",
  "socket2",