Parse NIL APPENDLIMIT values in STATUS responses

[OskarEichler]

Summary

Handle the NIL alternative of STATUS APPENDLIMIT explicitly. Numeric APPENDLIMIT already parses through generic number handling, but a mailbox reporting no limit currently raises ResponseParseError. Preserve numeric values and represent NIL as Ruby nil.

Reproduction

require 'net/imap'
p Net::IMAP::ResponseParser.new.parse("* STATUS INBOX (APPENDLIMIT NIL)\r\n").data.attr
# Before: ResponseParseError. After: {"APPENDLIMIT" => nil}.

Verification

• 60 focused checks, 24 failing expectations before and zero afterward. Keyword/NIL casing, zero/ordinary/maximum 32-bit numeric values, frozen inputs and surrounding attributes are covered. RFC7889 sections 3 and 5 specify NIL for no mailbox limit: https://www.rfc-editor.org/rfc/rfc7889.html#section-5
• Existing rake test on this isolated branch: 1726 tests, 12602 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications, Ruby 4.0.6 via rbenv. Baseline also passes 1,726 tests; assertion counts vary slightly between runs.
• Supplemental RuboCop Lint retains the same 48 existing findings. Syntax and git diff --check pass. No new/modified repository tests, dependencies or workflows; focused checks were external under the consumer repository's no-new-tests policy.
• Based on master 6d2ef7a636a1e2449187a83b06ac7a5baa54ead2; runtime differs from released 0.6.6 only in documentation before this change. Existing upstream PR searches found no matching fix.

Compatibility and limits

No signature, dependency or Ruby-minimum change. Valid APPENDLIMIT NIL responses now parse; numeric values retain Integer representation. This does not implement client-side APPEND size enforcement or change unknown extension parsing. No production or external IMAP service used. Other Ruby/OS versions were not run locally. Local success does not imply upstream CI approval or exhaustive coverage.

[nevans]

Similar to #748, can you add an example to test/net/imap/fixtures/response_parser/status_responses.yml, please?

If you leave out the :expected: value, the generated test will be pending and it will print out the appropriate :expected: for you to inspect, copy, and paste.

[OskarEichler]

Added the requested status_responses.yml fixture for APPENDLIMIT NIL. The full response-parser test file passes: 211 tests, 281 assertions.

[nevans]

Thanks!

[OskarEichler]

Upstream added the adjacent empty-STATUS fixture after the approval, which created a content conflict in the same YAML section. I merged current master, retained both fixtures without changing the APPENDLIMIT parser fix, and reran the focused response-parser suite: 212 tests, 282 assertions, all passing. GitHub dismissed the approval because of the merge commit, so this is ready for a quick re-approval when convenient.