Refactor src/stdlib/command.rs to resolve low cohesion

[coderabbitai]

Context

Related PR: #231
Requested by: @leynos

The current src/stdlib/command.rs file has grown to 1,022 lines with 47 functions spanning at least 9 different responsibilities, exceeding the cohesion threshold of 3. This issue tracks the refactoring work to split the module into focused sub-modules.

Current State

• File: src/stdlib/command.rs (1,022 lines, 47 functions, 9 responsibilities)
• Issue: Low cohesion detected by CodeScene analysis

Target

• Split into 8 sub-modules + coordinator under src/stdlib/command/
• Preserve all existing functionality, tests, and platform-specific behaviour
• Maintain existing public API surface via re-exports in mod.rs

Implementation Steps

1. Create Directory Structure

Create directory src/stdlib/command/ and move src/stdlib/command.rs to src/stdlib/command/mod_backup.rs

2. Create src/stdlib/command/config.rs

• Move: CommandConfig, OutputMode, OutputStream, PipeSpec, PipeLimit, CommandOptions (struct and impl blocks)
• Add derives/visibility as needed
• Keep all methods and Default impl for CommandOptions

3. Create src/stdlib/command/error.rs

• Move: CommandFailure enum with all variants and From<io::Error> impl
• Move: ExitDetails, LimitExceeded structs with impl blocks
• Move: All error constructor functions (spawn_error, io_error, broken_pipe_error, exit_error, timeout_error, output_limit_error, stream_path_error, command_error)
• Move: append_exit_status, append_stderr helper functions
• Import necessary types from super::config and super::context

4. Create src/stdlib/command/context.rs

• Move: CommandContext, GrepCall, CommandLocation structs with all impl blocks
• Import necessary types from super::config

5. Create src/stdlib/command/result.rs

• Move: StdoutResult, PipeOutcome enums

6. Create src/stdlib/command/execution.rs

• Move: run_command, run_program (cfg-gated), run_child, wait_for_exit
• Move: SHELL, SHELL_ARGS, COMMAND_TIMEOUT constants
• Import from super::{config, error, result, pipes, quote}

7. Create src/stdlib/command/pipes.rs

• Move: spawn_pipe_reader, join_reader, read_pipe, create_empty_tempfile
• Move: cleanup_readers, join_pipe_for_cleanup, handle_stdin_result
• Import from super::{config, error, result}

8. Create src/stdlib/command/filters.rs

• Move: execute_shell, execute_grep
• Move: collect_flag_args, format_command, to_bytes
• Import from super::{config, context, error, result, execution, quote}

9. Create src/stdlib/command/quote.rs

• Move: quote function (both Windows and non-Windows versions with cfg gates)
• Move: QuoteError enum with Display impl
• Move: The quote_escapes_cmd_metacharacters test under #[cfg(test)] mod tests
• Add necessary imports (fmt, shell_quote for non-Windows)

10. Create src/stdlib/command/mod.rs

• Add module documentation (move from original file header)
• Declare sub-modules: mod config; mod error; mod context; mod result; mod execution; mod pipes; mod filters; mod quote;
• Add necessary imports (Arc, AtomicBool, minijinja types, etc.)
• Move: register() function (import needed types from sub-modules)
• Add pub(crate) use statements to re-export types used by parent stdlib module
• Import value_from_bytes from super::value_from_bytes

11. Update Imports

• Ensure each sub-module imports only what it needs from siblings via super::module_name
• Use pub(crate) or pub(super) visibility appropriately
• Maintain platform-specific cfg gates for Windows/non-Windows code

12. Verification

• Run: make check-fmt
• Run: make lint
• Run: make test
• Ensure all tests pass and no functionality changed

Important Notes

• Keep platform-specific cfg attributes with their respective code
• Preserve all comments and documentation
• Use pub(crate) for items used outside the command module, pub(super) for items used between sub-modules
• The public API surface should remain identical via re-exports in mod.rs
• Each sub-module should have a single, clear responsibility, reducing the cohesion metric from 9 to 1–2 per module

Expected Outcome

After refactoring:

• Each sub-module focuses on a single responsibility
• Cohesion metric improves from 9 to 1–2 per module
• All existing tests pass
• Public API remains unchanged
• Code is more maintainable and easier to navigate