fix(ruby): classify missing consumers

[NikolayS]

Summary

• classify all SQL missing-consumer message variants as Pgque::ConsumerNotFound
• preserve the original PG::Error cause and backtrace
• cover direct, cooperative, and real-PostgreSQL error paths

Fixes #315.

Validation

• red regression: 3 runs, 3 assertions, 3 expected failures
• focused green: 3 runs, 20 assertions, 0 failures
• full Ruby suite: 79 runs, 212 assertions, 0 failures, 0 errors
• installed-gem classification smoke passed
• Ruby syntax and git diff --check passed

[NikolayS]

Real-user verification evidence

The original RED is retrospective because implementation and tests were committed together. Running the final classification test with the base library produced 3 runs, 3 assertions, 3 failures: each path raised generic Pgque::Error rather than Pgque::ConsumerNotFound.

PGQUE_TEST_DSN=postgresql:///pgque_pr_rules_audit \
  ruby -Iclients/ruby/lib -Iclients/ruby/test \
  clients/ruby/test/test_consumer_not_found.rb
PGQUE_TEST_DSN=postgresql:///pgque_pr_rules_audit ruby -S rake test
gem build pgque.gemspec --output /tmp/pgque-331-0.3.0.rc.1.gem

Focused head result: 3 runs, 20 assertions, 0 failures. Full database result: 79 runs, 212 assertions, 0 failures, 0 errors, 0 skips. An isolated installed gem confirmed Pgque::ConsumerNotFound < Pgque::Error and preserved the original PG::Error cause. Head CI: 17/17 checks passed.

[NikolayS]

REV rubric review

Automated ultrareview quota was unavailable, so I executed the five official
REV rubric prompts manually. SOC2 findings were omitted as instructed. This is
a comment-only review, not an approval.

Blocking findings

None. I found no unresolved functional, security, documentation, or test
blocker on head 36958af560593cdc698251c85b7713812833ea32.

Rubric results

• Bug hunter: all known missing-consumer server messages map to
  ConsumerNotFound; queue and batch classifications retain their ordering.
• Security reviewer: classification is local to trusted server error text and
  introduces no SQL, privilege, or information-disclosure path.
• Docs reviewer: this repairs behavior of an existing public exception class;
  no new setup or user workflow needs separate documentation.
• Guidelines checker: the PR is one surgical fix, title/commit use an allowed
  type, the subject is under 50 characters, and all CI checks pass.
• Test analyzer: direct, cooperative, and real-Postgres paths are covered;
  tests also assert preservation of the original PG::Error cause and
  backtrace. Recorded base failures and head successes provide RED/GREEN
  evidence.

Nonblocking findings

None from the manual rubrics. No code change is requested by this review.