Normalize response types consistently when clearing responses

[OskarEichler]

Summary

Normalize response types with to_s.upcase in clear_responses, matching responses(type) and extract_responses(type). Lowercase/mixed-case strings and symbols can currently read a response but silently fail to remove it.

Reproduction

require 'net/imap'
imap = Net::IMAP.allocate
imap.send(:mon_initialize)
imap.instance_variable_set(:@responses, {'EXISTS' => [3]})
p imap.responses(:exists) # [3]
p imap.clear_responses(:exists) # before: []; after: [3]

Verification

• 90 focused checks, 20 failing expectations before and zero afterward. Five string/symbol case variants, empty/nonempty response lists, actual key deletion, unrelated-response retention, frozen output and repeated clearing are covered. The original 💥 Changes to responses handling, motivated by thread-safety #93 introduced both methods; its source was reviewed.
• 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

Intentional correction: noncanonical type spellings now clear the corresponding canonical response key. Uppercase strings, nil/false whole-hash clearing and result freezing retain their existing behavior. This does not alter raw response-hash mutation policy. 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.