Move shared fixtures and parity utilities to TestUtilities project

- git mv JetStreamApiFixture, JetStreamClusterFixture, LeafFixture,
  Parity utilities, and TestData from NATS.Server.Tests to
  NATS.Server.TestUtilities
- Update namespaces to NATS.Server.TestUtilities (and .Parity sub-ns)
- Make fixture classes public for cross-project access
- Add PollHelper to replace Task.Delay polling with SemaphoreSlim waits
- Refactor all fixture polling loops to use PollHelper
- Add 'using NATS.Server.TestUtilities;' to ~75 consuming test files
- Rename local fixture duplicates (MetaGroupTestFixture,
  LeafProtocolTestFixture) to avoid shadowing shared fixtures
- Remove TestData entry from NATS.Server.Tests.csproj (moved to
  TestUtilities)

tests/NATS.Server.TestUtilities/TestData/full.conf

@@ -0,0 +1,57 @@
+# Full configuration with all supported options
+port: 4222
+host: "0.0.0.0"
+server_name: "full-test"
+client_advertise: "nats://public.example.com:4222"
+
+max_payload: 1mb
+max_control_line: 4096
+max_connections: 65536
+max_pending: 64mb
+write_deadline: "10s"
+max_subs: 0
+max_sub_tokens: 0
+max_traced_msg_len: 1024
+disable_sublist_cache: false
+max_closed_clients: 5000
+
+ping_interval: "2m"
+ping_max: 2
+
+debug: false
+trace: false
+trace_verbose: false
+logtime: true
+logtime_utc: false
+logfile: "/var/log/nats.log"
+log_size_limit: 100mb
+log_max_num: 5
+
+http_port: 8222
+http_base_path: "/nats"
+
+pidfile: "/var/run/nats.pid"
+ports_file_dir: "/var/run"
+
+lame_duck_duration: "2m"
+lame_duck_grace_period: "10s"
+
+server_tags {
+  region: "us-east"
+  env: "production"
+}
+
+authorization {
+  user: admin
+  password: "secret"
+  timeout: 2
+}
+
+tls {
+  cert_file: "/path/to/cert.pem"
+  key_file: "/path/to/key.pem"
+  ca_file: "/path/to/ca.pem"
+  verify: true
+  timeout: 2
+  handshake_first: true
+}