refactor: split compression tests into separate module files as planned

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/tests/mod.rs

@@ -0,0 +1,12 @@
+#[cfg(test)]
+pub mod compression;
+#[cfg(test)]
+pub mod compression_engine;
+#[cfg(test)]
+pub mod meta_plugin;
+#[cfg(test)]
+pub mod modes;
+#[cfg(test)]
+pub mod server;
+#[cfg(test)]
+pub mod common;