Fix email({ allowMultiple: true }) format/parse round-trip for quoted commas

[dahlia]

Summary

email({ allowMultiple: true }) could not round-trip values whose local parts contained quoted commas. format() joined addresses with a bare ,, producing output like "Doe, John"@example.com,x@example.com. While splitEmails() happened to handle this correctly due to its quote-tracking state machine, the bare comma join was fragile and not guaranteed to be unambiguous.

This PR changes format() to use , (comma-space) as the separator instead. This is safe because email addresses cannot start with whitespace, and parse() already trims each split result. It also matches the RFC 5322 conventional separator for address lists.

const parser = email({ allowMultiple: true });
const value = ['"Doe, John"@example.com', "x@example.com"];

// Before: "Doe, John"@example.com,x@example.com
// After:  "Doe, John"@example.com, x@example.com
parser.format(value);

Closes #354

Test plan

• Added round-trip test for quoted commas in local parts ("Doe, John"@example.com)
• Added round-trip test for escaped quotes with commas ("a\"b,c"@example.com)
• Updated existing format test in valueparser.test.ts to expect comma-space separator
• mise test passes across Deno, Node.js, and Bun

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.92%. Comparing base (8ae37e2) to head (1649271).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #626   +/-   ##
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  Coverage   94.92%   94.92%           
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  Files          38       38           
  Lines       17431    17431           
  Branches     4599     4600    +1     
=======================================
  Hits        16546    16546           
  Misses        873      873           
  Partials       12       12           

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 87ae74bb-aba0-4cb3-b6e7-d7f72bdbbd29

📥 Commits

Reviewing files that changed from the base of the PR and between d247413 and 1649271.

📒 Files selected for processing (3)

• CHANGES.md
• packages/core/src/valueparser.test.ts
• packages/core/src/valueparser.ts

---

Walkthrough

The pull request fixes an email formatting bug in the email({ allowMultiple: true }) parser. The issue was that multiple email addresses were being joined with a bare comma when formatted, causing round-trip failures for emails with quoted local parts containing commas (e.g., "Doe, John"@example.com). The fix changes the join separator from "," to ", " (comma followed by space) in the format method. Changes include a changelog entry documenting the fix, an updated test with new round-trip test cases, and a one-line code modification in the valueparser implementation.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

Possibly related PRs

• Fix email() splitting on commas inside quoted strings when allowMultiple is enabled #606: Modifies email value parser for allowMultiple addresses, likely addressing the complementary parsing logic to handle the new comma-space separator format.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: fixing email format/parse round-trip for quoted commas in allowMultiple mode.
Description check	✅ Passed	The description is directly related to the changeset, explaining the problem, solution, and test plan for the email round-trip fix.
Linked Issues check	✅ Passed	The PR successfully addresses issue #354 by changing format() to use comma-space separator, enabling round-trip parsing of quoted commas.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the email format/parse round-trip issue; no out-of-scope modifications were introduced.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-354-email-round-trip

📝 Coding Plan

• Generate coding plan for human review comments

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