Keep cookie jar storage separate from caller arrays

[OskarEichler]

Summary

Give CookieJar its own sorted array instead of sorting and retaining caller-owned storage.

Reproduction

require "rack/test"
p Rack::Test::CookieJar.new([].freeze).to_hash
# before: FrozenError; after: {}

Verification

• Based on main c9efe33c68112ed207d8ef9f90a28ecd95a727bf; Ruby 4.0.6 commands run through rbenv.
• Existing suite: 292 runs, zero failures/errors/skips, with Rack 3.2.7 and 2.2.24.
• 160 focused assertions cover original ordering, frozen input, later jar mutations and later caller mutations. This is separate from fix/cookie jar delete #356, whose rejected symbol-conversion proposal is not included.
• History.md updated. Source/History-only patch: no test files added or changed under the consuming repository's no-test-change policy; the executable reproduction is above.
• No full Ruby/JRuby/platform matrix claim; existing remote CI remains the matrix check.

Breaking changes / limitations

No cookie matching/protocol changes. The caller array is no longer sorted or modified by later jar operations; applications relying on that undocumented side effect will observe a change.