Normalize EHLO capability keywords without changing parameters

[OskarEichler]

Summary

Normalize EHLO extension keywords to uppercase while retaining parameter values. Lowercase/mixed-case replies currently leave capability lookups empty, including SMTPUTF8; RFC 5321 section 4.1.1.1 requires case-insensitive keyword handling.

Reproduction

require 'net/smtp'
response = Net::SMTP::Response.parse("250-fixture\r\n250-smtputf8\r\n250 size 12345\r\n")
p response.capabilities
# Before: {"smtputf8"=>[], "size"=>["12345"]}
# After:  {"SMTPUTF8"=>[], "SIZE"=>["12345"]}

Verification

• 38 focused checks cover four keyword case variants, six extensions, parameter case preservation, frozen response input, existing single-line/empty-keyword behavior and a bounded localhost SMTPUTF8 sender command. Twenty-four failed expectations before; zero after. Existing Allow case-insensitive strings for SASL mechanism #64 normalizes requested authentication mechanism names; it does not handle EHLO response keywords. Standard: https://www.rfc-editor.org/rfc/rfc5321.html#section-4.1.1.1
• Existing rake test: 95 tests, 170 assertions, zero failures/errors, on baseline and this isolated branch, Ruby 4.0.6 via rbenv. Existing local TLS cases are included.
• Supplemental RuboCop Lint has the same six existing findings; Ruby syntax and git diff --check pass. No new/modified repository tests, dependencies or workflow settings.
• Independent branch based on c2e9102e1f199a5a10e5261a776d752e64cc130f; runtime matches released 0.5.1. External focused checks were used under the consumer repository's no-new-tests policy.

Compatibility and limits

No signature/dependency change. Capability hash keys are now canonical uppercase even when the server uses lowercase/mixed case. Code explicitly looking up a lowercase server spelling must use the canonical key. Parameters, including SASL mechanism names, are not case-folded. This is not a new malformed-response validation policy. Other Ruby/OS runtimes were not run locally. No production or external SMTP delivery was used. Maintainer CI status is separate from these local checks.