fix: fail with a clear error when LD_SDK_KEY is unset

[devin-ai-integration]

Summary

Running the example without LD_SDK_KEY crashed with a multi-thousand-line supervisor report (the whole TLS options map dumped twice), because os:getenv/1 returns the atom false — not "" — when the variable is unset, and that atom went straight into ldclient:start_instance/2.

• hello_erlang_server:init/1 now returns {stop, "LD_SDK_KEY is not set. ..."} for a missing or empty key, matching the hello-elixir example's behaviour.
• Boot then fails fast with a readable one-line reason instead of an unreadable crash dump.
• No dependency changes: ldclient 3.11.2 is the latest release in the current major, and rebar3 upgrade --all reports every locked transitive dependency already at its latest resolvable version.
• No deprecated SDK APIs and no deprecation warnings when compiling or running.

Implementation details

init(_Args) ->
  case os:getenv("LD_SDK_KEY") of
    SdkKey when SdkKey =:= false; SdkKey =:= "" ->
      {stop, "LD_SDK_KEY is not set. Set it to your LaunchDarkly server-side SDK key and try again."};
    SdkKey ->
      ldclient:start_instance(SdkKey, #{http_options => #{tls_options => ldclient_config:tls_basic_options()}}),
      {ok, []}
  end.

An earlier revision of this PR started the client in offline mode instead; per review feedback it now fails loudly, since a silently-offline example that always returns fallbacks is more confusing than a startup error.

Testing (Erlang/OTP 27 via docker run erlang:27, rebar3 compile + rebar3 shell):

• With a real server SDK key: streaming connection established, hello_erlang_server:get(<<"hello-boolean">>, false, <<"user-key-123abc">>) returned the flag value from LaunchDarkly.
• With LD_SDK_KEY unset: ===> Failed to boot hello_erlang for reason {{shutdown,{failed_to_start_child,console,"LD_SDK_KEY is not set. ..."}}, ...}.

Not a UI change, so no screenshots or staging link apply.

Link to Devin session: https://app.devin.ai/sessions/39c567ec09c14cb3973cb2f73337f733
Open in Devin Desktop: https://app.devin.ai/desktop/session/39c567ec09c14cb3973cb2f73337f733?variant=devin

[devin-ai-integration]

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[devin-ai-integration]

@cursor review

[kinyoklion]

I would prefer failing with a clear error like the elixir example.

[devin-ai-integration]

Done in 6335ee3 — init/1 now returns {stop, "LD_SDK_KEY is not set. Set it to your LaunchDarkly server-side SDK key and try again."}, same as the Elixir example, instead of starting offline. Boot output with the key unset:

===> Failed to boot hello_erlang for reason {{shutdown,
                                              {failed_to_start_child,console,
                                               "LD_SDK_KEY is not set. Set it to your LaunchDarkly server-side SDK key and try again."}},
                                             {hello_erlang_app,start,[normal,[]]}}

Re-verified with a real SDK key that the flag still evaluates.