refactor: use release-please markers for version instead of Application.spec (#51)

Use x-release-please markers directly in the code to maintain a
pseudo-hardcoded version number, rather than reading from Application.spec
at runtime. This approach:

- Makes the implementation consistent across Flagsmith SDKs
- Relies solely on release-please for version management
- Keeps version visible in source code for easier debugging
- Simplifies the user_agent/0 function implementation

The version is now defined directly in lib/flagsmith_client.ex between
x-release-please markers, and release-please will automatically update
it during releases.

Also removed the redundant user_agent/0 unit test since the function now
simply returns a hardcoded string with no conditional logic. The HTTP
integration test sufficiently validates the User-Agent header behavior.

Co-authored-by: Claude <claude@anthropic.com>

Author: emyller

lib/flagsmith_client.ex

@@ -11,10 +11,10 @@ defmodule Flagsmith.Client do
   @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
+    # x-release-please-start-version
+    version = "2.3.0"
+    # x-release-please-end
+    "flagsmith-elixir-sdk/#{version}"
   end
 
   @doc """

release-please-config.json

@@ -10,6 +10,7 @@
             "prerelease": false,
             "include-component-in-tag": false,
             "extra-files": [
+                "lib/flagsmith_client.ex",
                 "test/flagsmith_client_test.exs"
             ]
         }

test/flagsmith_client_test.exs

@@ -439,15 +439,6 @@ defmodule Flagsmith.Client.Test do
   end
 
   describe "User-Agent header" do
-    test "user_agent/0 returns expected version" do
-      # x-release-please-start-version
-      expected_version = "2.3.0"
-      # x-release-please-end
-
-      user_agent = Flagsmith.Client.user_agent()
-      assert user_agent == "flagsmith-elixir-sdk/#{expected_version}"
-    end
-
     test "HTTP client includes User-Agent header", %{config: config} do
       # x-release-please-start-version
       expected_version = "2.3.0"