Preserve numeric create and truncate flags in v5 open requests

[OskarEichler]

Summary

Calculate the creation disposition independently of read/write access and encode TRUNCATE_EXISTING when TRUNC is supplied without CREAT.

Reproduction

driver.open(path, IO::WRONLY | IO::TRUNC, {}) currently sends OPEN_EXISTING, retaining old contents. Fixed sends TRUNCATE_EXISTING. Read-only CREAT/EXCL flags also retain their requested creation behavior without gaining write-data access.

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.
• 288 external assertions cover both v5/v6, all three access modes, and every combination of CREAT/EXCL/TRUNC/APPEND. Existing string-mode fixtures continue passing. This follows the v5 open disposition layout.
• 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 behavior change: explicitly requested create/truncate flags now take effect, including truncating an existing file when requested. Callers depending on ignored flags must remove those flags. v1-v4 and public signatures are unchanged. No native v5/v6 server was available; verification is packet-level.

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.