Preserve server status messages starting with protocol v3

[OskarEichler]

Summary

Decode the message and language tag so Response and StatusException can use server-provided diagnostics. Preserve v1/v2 and existing code-only response compatibility.

Reproduction

Parse Net::SSH::Buffer.from(:long, 4, :string, "Synthetic disk full", :string, "en") with a v3 driver. Baseline returns only {code: 4}; fixed also retains message and language. Addresses the diagnostic concern in #156.

Verification

• Baseline and this isolated patch: 433 existing tests, 1,201 assertions, zero failures/errors/skips, Ruby 4.0.6 through rbenv, net-ssh 7.3.3, Minitest 5.27.0 and Mocha 2.1.0.
• 120 external assertions cover v1-v6, code-only packets, empty/nonempty/UTF-8 messages and unknown codes. A missing-file error from a local OpenSSH SFTP process now carries its server message. Message/language were introduced in protocol v3.
• The combined consumer candidate also passes 10,384 external model assertions, 21 local OpenSSH pipe assertions, and the existing suite with frozen string literals enabled and disabled. These combined checks include separately attributed portions of Handle frozen strings #157/ensure ruby 4 compatibility and address some security concerns #162; they are not claimed as this isolated branch's changes.
• Packaging preserves all 37 release paths and dependency metadata. All 26 runtime files compile on Ruby 4.0.6 and parse with Parser::Ruby25. Comparative Lint checks retain the same 33 baseline findings; no all-green lint claim.

Breaking changes and limitations

Intentional observable change: status/error strings can now include the actual server message, as already supported by Response. Numeric status codes are unchanged. The raw fields are preserved without translating text. v5/v6 error-specific extension payloads remain outside this patch.

No repository tests were added or modified under the contribution's no-new-tests constraint; focused repro/model drivers ran outside the repository. An external verification Gemfile supplies compatible test tools and RDoc without changing upstream tooling. Actual older Ruby/JRuby/Windows compatibility and upstream CI have not been verified. No production or SSH connections, version bumps, release-tool changes or unrelated modernization are included. Git builds are not signed by the upstream gem release key.