feat: implement IAccountResolver interface and MemAccountResolver

Adds the IAccountResolver interface (FetchAsync, StoreAsync, IsReadOnly)
and a MemAccountResolver backed by ConcurrentDictionary for in-memory
JWT storage in tests and simple operator deployments.

Reference: golang/nats-server/server/accounts.go:4035+

tests/NATS.Server.Tests/AccountResolverTests.cs

@@ -0,0 +1,68 @@
+using NATS.Server.Auth.Jwt;
+
+namespace NATS.Server.Tests;
+
+public class AccountResolverTests
+{
+    [Fact]
+    public async Task Store_and_fetch_roundtrip()
+    {
+        var resolver = new MemAccountResolver();
+        const string nkey = "AABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQ";
+        const string jwt = "eyJhbGciOiJlZDI1NTE5LW5rZXkiLCJ0eXAiOiJKV1QifQ.payload.sig";
+
+        await resolver.StoreAsync(nkey, jwt);
+        var fetched = await resolver.FetchAsync(nkey);
+
+        fetched.ShouldBe(jwt);
+    }
+
+    [Fact]
+    public async Task Fetch_unknown_key_returns_null()
+    {
+        var resolver = new MemAccountResolver();
+
+        var result = await resolver.FetchAsync("UNKNOWN_NKEY");
+
+        result.ShouldBeNull();
+    }
+
+    [Fact]
+    public async Task Store_overwrites_existing_entry()
+    {
+        var resolver = new MemAccountResolver();
+        const string nkey = "AABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQ";
+        const string originalJwt = "original.jwt.token";
+        const string updatedJwt = "updated.jwt.token";
+
+        await resolver.StoreAsync(nkey, originalJwt);
+        await resolver.StoreAsync(nkey, updatedJwt);
+        var fetched = await resolver.FetchAsync(nkey);
+
+        fetched.ShouldBe(updatedJwt);
+    }
+
+    [Fact]
+    public void IsReadOnly_returns_false()
+    {
+        IAccountResolver resolver = new MemAccountResolver();
+
+        resolver.IsReadOnly.ShouldBeFalse();
+    }
+
+    [Fact]
+    public async Task Multiple_accounts_are_stored_independently()
+    {
+        var resolver = new MemAccountResolver();
+        const string nkey1 = "AABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQ1";
+        const string nkey2 = "AABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789ABCDEFGHIJKLMNOPQ2";
+        const string jwt1 = "jwt.for.account.one";
+        const string jwt2 = "jwt.for.account.two";
+
+        await resolver.StoreAsync(nkey1, jwt1);
+        await resolver.StoreAsync(nkey2, jwt2);
+
+        (await resolver.FetchAsync(nkey1)).ShouldBe(jwt1);
+        (await resolver.FetchAsync(nkey2)).ShouldBe(jwt2);
+    }
+}