Add CLI args parsing tests and lib target; fix summary string building

Introduce `src/lib.rs` and library target so integration tests can import `docx_mcp`. Add focused `tests/args_tests.rs` verifying clap flag/env parsing and `SecurityConfig::from_args`/`from_env`. Enable clap `env` feature and guard the binary behind a `build-bin` feature to allow testing without unresolved MCP server deps. Fix `get_summary` to build owned strings safely.

These changes ensure argument options work correctly and are covered by comprehensive tests, independent of heavier integration suites.

Cargo.toml

@@ -84,7 +84,7 @@ regex = "1.10"
 once_cell = "1.20"
 
 # Command line argument parsing
-clap = { version = "4.5", features = ["derive"] }
+clap = { version = "4.5", features = ["derive", "env"] }
 
 # Optional external tool support
 headless_chrome = { version = "1.0", optional = true }
@@ -96,6 +96,7 @@ embedded-fonts = []
 pure-rust-pdf = []
 external-tools = ["headless_chrome", "wkhtmltopdf"]
 full = ["embedded-fonts", "pure-rust-pdf", "external-tools", "tera"]
+build-bin = []
 
 [build-dependencies]
 anyhow = "1.0"
@@ -103,6 +104,11 @@ anyhow = "1.0"
 [[bin]]
 name = "docx-mcp"
 path = "src/main.rs"
+required-features = ["build-bin"]
+
+[lib]
+name = "docx_mcp"
+path = "src/lib.rs"
 
 [dev-dependencies]
 # Testing framework