fix(gmail): use case-insensitive matching for email headers in parse_message_headers

[gura105]

Description

parse_message_headers uses exact case-sensitive string matching for header names ("Cc", "From", etc.), but the Gmail API preserves original header casing from the sending MTA. Per RFC 5322 §1.2.2, header field names are case-insensitive.

This causes +reply-all (and other commands) to silently drop CC recipients when the original message was sent through an MTA that uses non-canonical casing (e.g., Microsoft Exchange emits "CC" instead of "Cc").

Root cause

// Before: exact match — "CC" falls through to _ => {}
match name {
    "Cc" => append_address_list_header_value(&mut parsed.cc, value),
    ...
}

Fix

Use eq_ignore_ascii_case match guards for zero-allocation case-insensitive matching, consistent with the existing get_part_header and extract_header functions in the same file.

// After: case-insensitive — "CC", "Cc", "cc" all match
match name {
    s if s.eq_ignore_ascii_case("cc") => {
        append_address_list_header_value(&mut parsed.cc, value)
    }
    ...
}

This also simplifies the Message-ID arm from "Message-ID" | "Message-Id" to just eq_ignore_ascii_case("message-id").

Scope

All headers in the match block are affected, not just Cc:

Header	Impact if missed
From	Hard error ("Message is missing From header")
Message-ID	Hard error ("Message is missing Message-ID header")
To	+reply-all misses To recipients
CC	+reply-all drops CC recipients
Reply-To	Reply goes to wrong recipient
References	Threading breaks

Tests

3 new tests (701 total, up from 698):

• test_parse_message_headers_all_uppercase — all-caps headers (Exchange/Outlook pattern)
• test_parse_message_headers_all_lowercase — all-lowercase headers
• test_parse_message_headers_mixed_case — canonical + non-canonical mix

All existing tests pass unchanged.

Fixes #642

Note: This is a resubmission of #643, which was auto-closed due to 72h inactivity.

Checklist:

• My code follows the AGENTS.md guidelines (no generated google-* crates).
• I have run cargo fmt --all to format the code perfectly.
• I have run cargo clippy -- -D warnings and resolved all warnings.
• I have added tests that prove my fix is effective or that my feature works.
• I have provided a Changeset file (e.g. via pnpx changeset) to document my changes.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5d071a7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@googleworkspace/cli	Patch

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[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where email header parsing was failing due to strict case-sensitive matching. By adopting case-insensitive comparisons, the system now correctly handles headers regardless of the originating MTA's casing conventions, ensuring reliable extraction of essential message metadata.

Highlights

• Case-Insensitive Header Matching: Updated parse_message_headers to use eq_ignore_ascii_case for matching email header names, ensuring compatibility with various MTAs that may use non-canonical casing.
• Improved Robustness: Prevents silent data loss for critical headers like CC, To, and Reply-To when interacting with the Gmail API.
• Expanded Test Coverage: Added three new test cases to verify header parsing across all-uppercase, all-lowercase, and mixed-case scenarios.

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[gemini-code-assist]

Code Review

This pull request implements case-insensitive matching for email headers in the parse_message_headers function within the Gmail helper module, aligning with RFC 5322 requirements. The changes replace exact string matching with eq_ignore_ascii_case for standard headers and include new unit tests covering uppercase, lowercase, and mixed-case scenarios. I have no feedback to provide.