build: enable ppx_inline_test preprocessing and inline tests

Co-authored-by: aider (openai//workspace/models/openai-mirror/gpt-oss-120b/) <aider@aider.chat>

Author: zenfey

lib/dune

@@ -1,3 +1,5 @@
 (library
  (name dune_ai_context)
- (libraries str unix compiler-libs.common sexplib ppx_inline_test))
+ (libraries str unix compiler-libs.common sexplib ppx_inline_test)
+ (preprocess (pps ppx_inline_test))
+ (inline_tests true))