feat(node:process): implement process.initgroups

[ddmoney420]

Summary

Implements the process.initgroups(user, extraGroup) function for Node.js compatibility.

• Takes a username string as the first argument
• Takes an extra group (number or group name string) as the second argument
• Validates the user exists before calling initgroups
• Throws appropriate errors for invalid arguments:
  • ERR_INVALID_ARG_TYPE if user is not a string
  • ERR_UNKNOWN_CREDENTIAL if user doesn't exist
  • ERR_UNKNOWN_CREDENTIAL if group doesn't exist (when passed as string)
• Only available on non-Windows platforms (consistent with Node.js behavior)

Closes #23891

Test Plan

• Added tests verifying:
  • process.initgroups is a function
  • Throws with invalid user type (number instead of string)
  • Throws with unknown user
  • Is undefined on Windows

AI Disclosure

This PR was written primarily by Claude Code.

🤖 Generated with Claude Code

[coderabbitai]

Walkthrough

Implementation of the process.initgroups function for POSIX systems, including hostname and group ID validation with corresponding test coverage for both POSIX and Windows platforms.

Changes

Cohort / File(s)	Summary
Process.initgroups implementation src/bun.js/bindings/BunProcess.cpp	Added host function Process_functioninitgroups with username validation, user lookup via getpwnam_r, gid resolution, and system call to initgroups. Registered in public API with arity 2.
Process.initgroups tests test/js/node/process/process.test.js	Added tests validating process.initgroups exists on POSIX platforms with checks for invalid input types and unknown users. Updated Windows tests to assert initgroups is undefined.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'feat(node:process): implement process.initgroups' clearly and specifically describes the main change: implementing the process.initgroups function for Node.js compatibility.
Description check	✅ Passed	The PR description is comprehensive, covering the summary, implementation details, error handling, platform availability, test plan, and includes required sections about what the PR does and verification approach.
Linked Issues check	✅ Passed	The PR successfully implements process.initgroups with all requirements from issue #23891: the function is now available on POSIX platforms, accepts username and extra group arguments, validates inputs correctly, and throws appropriate errors.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing process.initgroups: one implementation file adds the host function, and the test file adds appropriate test coverage for the new functionality.

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[claude]

Node.js compatibility issue: According to Node.js documentation, process.initgroups(user, extraGroup) accepts both string | number for the user parameter (numeric UID).

The current implementation only accepts strings. Node.js handles numeric UIDs by looking up the username via getpwuid_r() before calling initgroups().

For reference, the extraGroup parameter already uses maybe_gid_by_name() to handle both types. A similar approach would provide full compatibility.