feat: add User-Agent header with SDK version

- Add user_agent/0 function to dynamically retrieve SDK version from Application.spec
- Format: flagsmith-elixir-sdk/<version> or flagsmith-elixir-sdk/unknown as fallback
- Add user_agent_middleware/0 to inject User-Agent header in all HTTP requests
- Add comprehensive tests for User-Agent header functionality
- Follows the pattern established in Go and Java Flagsmith SDKs

Co-authored-by: Evandro Myller <evandro.myller@flagsmith.com>
Signed-off-by: Evandro Myller <evandro.myller@flagsmith.com>

Fixes #49

Author: claude

lib/flagsmith_client.ex

@@ -8,6 +8,15 @@ defmodule Flagsmith.Client do
   @type tesla_header_list :: [{String.t(), String.t()}]
   @type config_or_env :: Configuration.t() | Keyword.t() | Schemas.Environment.t()
 
+  @doc false
+  @spec user_agent() :: String.t()
+  def user_agent do
+    case Application.spec(:flagsmith_engine, :vsn) do
+      nil -> "flagsmith-elixir-sdk/unknown"
+      vsn -> "flagsmith-elixir-sdk/#{vsn}"
+    end
+  end
+
   @doc """
   Create a `t:Flagsmith.Configuration.t/0` struct with the desired settings to use
   in requests.
@@ -31,6 +40,7 @@ defmodule Flagsmith.Client do
     Tesla.client([
       base_url_middleware(api_url),
       auth_middleware(environment_key),
+      user_agent_middleware(),
       Tesla.Middleware.JSON,
       {Tesla.Middleware.Retry, max_retries: retries},
       {Tesla.Middleware.Headers, custom_headers},
@@ -394,6 +404,12 @@ defmodule Flagsmith.Client do
   def auth_middleware(environment_key),
     do: {Tesla.Middleware.Headers, auth_header(environment_key)}
 
+  @doc false
+  @spec user_agent_middleware() ::
+          {Tesla.Middleware.Headers, tesla_header_list()}
+  def user_agent_middleware(),
+    do: {Tesla.Middleware.Headers, [{"user-agent", user_agent()}]}
+
   @doc false
   @spec base_url_middleware(base_url :: String.t()) ::
           {Tesla.Middleware.BaseUrl, String.t()}

test/flagsmith_client_test.exs

@@ -437,4 +437,37 @@ defmodule Flagsmith.Client.Test do
                Flagsmith.Client.get_flag(new_config, "some_flag")
     end
   end
+
+  describe "User-Agent header" do
+    test "user_agent/0 returns version from application spec" do
+      user_agent = Flagsmith.Client.user_agent()
+      
+      # Should match the format "flagsmith-elixir-sdk/<version>" or "flagsmith-elixir-sdk/unknown"
+      assert user_agent =~ ~r/^flagsmith-elixir-sdk\/.+$/
+      
+      # In test environment, it should return the actual version from mix.exs
+      # or "unknown" if not available
+      case Application.spec(:flagsmith_engine, :vsn) do
+        nil -> assert user_agent == "flagsmith-elixir-sdk/unknown"
+        vsn -> assert user_agent == "flagsmith-elixir-sdk/#{vsn}"
+      end
+    end
+
+    test "HTTP client includes User-Agent header in requests", %{config: config} do
+      expect(Tesla.Adapter.Mock, :call, fn tesla_env, _options ->
+        # Assert that the User-Agent header is present
+        user_agent_header = Enum.find(tesla_env.headers, fn {header, _} ->
+          header == "user-agent"
+        end)
+        
+        assert user_agent_header != nil, "User-Agent header should be present"
+        {_header, user_agent_value} = user_agent_header
+        assert user_agent_value =~ ~r/^flagsmith-elixir-sdk\/.+$/
+        
+        {:ok, %Tesla.Env{status: 200, body: Test.Generators.map_env()}}
+      end)
+
+      Flagsmith.Client.get_environment(config)
+    end
+  end
 end