Support Elixir 1.18 JSON module as json_codec

[donleandro]

Summary

Fixes #299.

When configuring json_codec: JSON to use Elixir 1.18's built-in JSON module, calls to JSON.encode!/2 fail because the built-in module expects an encoder function as the second argument, while this library passes a keyword list of options (the convention used by Jason).

This came up while upgrading to Elixir 1.18 on a project I work on (Shiko, a veterinary platform). The fix is minimal and fully backward compatible:

• safe_encode!/3 in Absinthe.Plug: when opts is [] (which is the default when json_codec is configured as a bare module), it calls encode!/1 instead of encode!/2. Both Jason and JSON support arity-1. When opts is non-empty, it passes them through as before, so {Jason, [pretty: true]} etc. still works.
• pretty_encode!/2 in Absinthe.Plug.GraphiQL: the GraphiQL interface calls encode!(value, pretty: true) for display purposes. Since the built-in JSON module does not support this keyword option, the helper checks whether the codec supports keyword-style opts and falls back to plain encoding when it does not.

No changes to the public API. Jason and Poison keep working exactly as before.

Changes

• lib/absinthe/plug.ex - encode/4 and encode_json!/2 now go through safe_encode!/3
• lib/absinthe/plug/graphiql.ex - all encode!(value, pretty: true) calls go through pretty_encode!/2
• Tests added for safe_encode!/3 with both Jason and JSON, plus integration tests using json_codec: JSON for full request/response cycles (guarded with Code.ensure_loaded?(JSON) so they only run on Elixir >= 1.18)

Test plan

• All 91 tests pass (86 existing + 5 new)
• Verified with json_codec: JSON on Elixir 1.18.4
• Verified Jason still works identically (default behavior)
• New tests are skipped gracefully on Elixir < 1.18