Enforce -I mode argument size limits for xargs

[Copilot]

POSIX requires xargs to enforce size limits on constructed arguments in insert mode (-I). Current implementation lacks this validation.

Changes

Insert Mode Argument Size Limits

• Added INSERT_ARG_MAX constant (4096 bytes, exceeds POSIX minimum of 255)
• Validates constructed arguments in exec_insert_mode() using iterator methods
• Returns descriptive error when limit exceeded

Test Coverage

Added 8 integration tests covering:

• Insert mode size limit enforcement
• Single and mixed quote handling
• Empty input, line continuation
• Multiple replstr occurrences
• Combined -n and -s options
• Five arguments with replstr (POSIX minimum)

Test count: 17 → 25 (all passing)

Example

Added validation in insert mode:

// Check that constructed arguments don't exceed the limit
if let Some(arg) = util_args.iter().find(|arg| arg.len() > INSERT_ARG_MAX) {
    return Err(io::Error::new(
        io::ErrorKind::InvalidInput,
        format!(
            "xargs: constructed argument of {} bytes exceeds {} byte limit in insert mode",
            arg.len(),
            INSERT_ARG_MAX
        ),
    ));
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Audit xargs util</issue_title>
<issue_description>Review the xargs POSIX spec, xargs.html, analyze our implementation (process/xargs.rs, process/tests/xargs/mod.rs), identify any gaps in implementation or tests. If found, plan and implement gap remedies.</issue_description>

<agent_instructions>review CONTRIBUTING.md. cargo fmt required to pass CI/CD (which runs cargo tests on multiple arch's).</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Audit xargs util #470

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[jgarzik]

Request changes:

1. Remove the code >= 128 signal handling (all 3 locations in the macro and
   the two manual handlers)
2. Keep the INSERT_ARG_MAX constant and validation
3. Keep all the tests (they test valid behaviors)

[Copilot]

Request changes:

1. Remove the code >= 128 signal handling (all 3 locations...

Done in commit 99df609. Removed all signal handling code (code >= 128) from the macro and two manual handlers. Kept INSERT_ARG_MAX constant and validation, and all 25 tests continue to pass.