Restore message logging and clear write state on exceptional exits

[OskarEichler]

Summary

Restore temporarily disabled message logging and clear temporary write buffers/counters with ensure when reads, sources, writes or user blocks exit exceptionally. Do not finish or send the message terminator on failure.

Reproduction

require 'net/protocol'
require 'stringio'
debug, wire = StringIO.new, StringIO.new
io = Net::InternetMessageIO.new(wire, debug_output: debug)
begin
  io.write_message_by_block { |w| w << 'partial'; raise 'stop' }
rescue RuntimeError
end
p io.debug_output.equal?(debug)         # before: false; after: true
p io.instance_variable_get(:@wbuf)      # before: "partial"; after: nil
p io.instance_variable_get(:@written_bytes) # before: 0; after: nil
p wire.string                          # empty before and after

Focused checks cover reader EOF and block exceptions, nonlocal read exits, failing enumerable sources and IO writes, nonlocal writer breaks, successful empty/dot-stuffed/unterminated messages, exact byte counts and ordinary BufferedIO write failure. Original exceptions/return values are preserved.

Verification

• Ruby 4.0.6 via rbenv; reviewed master 6cba9756b72eaa5a939b4cb6e032ec1da88b07bd.
• Existing bundle exec rake test: 30 tests, 66 assertions, zero failures/errors on the baseline and this branch.
• cleanup: 57 checks, 0 failures in an external scratch script; baseline fails the affected expectations.
• Ruby syntax and git diff --check pass. Supplemental RuboCop Lint has the same 11 pre-existing offenses; no lint-clean claim.
• No repository tests added or modified, following the consuming project's explicit no-new-tests instruction. The reproduction is included here for maintainer use.
• Existing PRs/issues were checked for overlap, including Improve BufferedIO performance #14, Fix Net::ReadTimeout errors in net-pop by handling multi-byte terminators getting split up in readuntil #30, Fix readuntil missing a terminator that spans two reads #66 and Add limit option to Net::BufferedIO#readuntil #67. This change does not replace their buffering/terminator/read-limit fixes.

Compatibility and limitations

No signature changes or successful wire-format changes. Logging is restored after failure, and failed calls no longer retain partial message data/counters. This cleanup does not make a partially failed protocol exchange safe to resume; callers must retain their existing connection-disposal behavior.

The patch also applies to installed release 0.3.0, whose runtime file matches the reviewed master. Gem version, dependencies and Ruby >= 2.6 requirement stay unchanged. Only macOS/Ruby 4.0.6 was run locally; other Ruby/OS combinations require upstream CI. No production traffic or external service was used.