Preserve caller SSL context options when starting SMTP

[OskarEichler]

Summary

Copy ssl_context_params before adding the default verify_mode. Starting a session must not mutate a caller-owned option hash, and frozen hashes should remain usable. This preserves the existing verification defaults and explicit overrides.

Reproduction

require 'net/smtp'
smtp = Net::SMTP.new('example.invalid', starttls: false)
options = {}.freeze
smtp.ssl_context_params = options
def smtp.do_start(*) = nil  # keep this reproduction offline
def smtp.do_finish = nil
p smtp.start { :started }
# Before: FrozenError while inserting :verify_mode.
# After: :started; options remains empty and frozen.

Verification

• 50 focused checks cover frozen/mutable input, existing verify_mode values, both tls_verify settings, option reference retention and a bounded localhost session with frozen empty options. Baseline: seven failed expectations; patched: zero.
• 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, Ruby minimum or TLS policy change. The intentional behavior correction is that caller option hashes no longer acquire :verify_mode. This is a shallow copy; option values retain their existing treatment. 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.