docs: add plan for refactoring tests.rs into multiple files

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

PLAN.md

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+# Test Refactoring Plan
+
+This document outlines the steps to refactor `tests.rs` into multiple files and expand test coverage across the codebase.
+
+## Phase 1: Refactor Existing Tests
+
+1. **Split compression tests into separate module files**
+   - Files to update:
+     - `src/tests.rs` (remove compression_tests module)
+   - Files to add:
+     - `src/tests/compression/mod.rs`
+     - `src/tests/compression/gzip_tests.rs`
+     - `src/tests/compression/lz4_tests.rs`
+     - `src/tests/compression/none_tests.rs`
+
+2. **Split compression type tests into separate module files**
+   - Files to update:
+     - `src/tests.rs` (remove compression_type_tests module)
+   - Files to add:
+     - `src/tests/compression/types/mod.rs`
+     - `src/tests/compression/types/factory_tests.rs`
+     - `src/tests/compression/types/conversion_tests.rs`
+
+3. **Create test module structure**
+   - Files to add:
+     - `src/tests/mod.rs` (main test module file)
+     - `src/tests/common/mod.rs`
+
+## Phase 2: Add Tests for Other Modules
+
+4. **Add database tests**
+   - Files to add:
+     - `src/tests/db/mod.rs`
+     - `src/tests/db/item_tests.rs`
+     - `src/tests/db/tag_tests.rs`
+     - `src/tests/db/meta_tests.rs`
+
+5. **Add meta plugin tests**
+   - Files to add:
+     - `src/tests/meta_plugin/mod.rs`
+     - `src/tests/meta_plugin/system_tests.rs`
+     - `src/tests/meta_plugin/digest_tests.rs`
+     - `src/tests/meta_plugin/program_tests.rs`
+
+6. **Add compression engine tests**
+   - Files to add:
+     - `src/tests/compression_engine/mod.rs`
+     - `src/tests/compression_engine/program_tests.rs`
+
+7. **Add mode tests**
+   - Files to add:
+     - `src/tests/modes/mod.rs`
+     - `src/tests/modes/save_tests.rs`
+     - `src/tests/modes/get_tests.rs`
+     - `src/tests/modes/list_tests.rs`
+     - `src/tests/modes/delete_tests.rs`
+     - `src/tests/modes/update_tests.rs`
+     - `src/tests/modes/info_tests.rs`
+     - `src/tests/modes/status_tests.rs`
+     - `src/tests/modes/diff_tests.rs`
+
+8. **Add server tests**
+   - Files to add:
+     - `src/tests/server/mod.rs`
+     - `src/tests/server/api_tests.rs`
+     - `src/tests/server/auth_tests.rs`
+
+9. **Add utility tests**
+   - Files to add:
+     - `src/tests/common/is_binary_tests.rs`
+     - `src/tests/common/status_tests.rs`
+
+10. **Update main test module**
+    - Files to update:
+      - `src/tests/mod.rs` (add all test module declarations)