Enforce exception message conventions and run lint separately

[dblock]

Summary

• Add rubocop-exception_messages and modern RuboCop to the main Gemfile, conditionally on Ruby 3.2+. No separate Gemfile.
• Run lint in the dedicated Ruby 3.3 job and tests across the unchanged supported-Ruby matrix. Default rake runs specs only.
• Correct exception messages and regression expectations, excluding generated API code.
• Run rubocop -a once and retain safe corrections; baseline only three remaining unrelated offenses.
• Add Enforce exception message conventions and run lint separately #603 to CHANGELOG and document workflow conventions in AGENTS.md.

Validation

• RuboCop: 75 files, no offenses.
• Specs excluding the CLI file: 619 examples, no failures.
• Ruby 2.7 Gemfile evaluation excludes all lint dependencies.
• Four CLI cases previously failed locally because an existing missing-bigdecimal-extension Bundler warning contaminates subprocess output.
• Existing local slack-api-ref submodule changes are excluded.

Prepared by an AI on behalf of @dblock.

[github-actions]

Danger Report

No issues found.

View run

[coveralls]

Coverage Report for CI Build 37309723822

Coverage decreased (-0.003%) to 91.046%

Details

• Coverage decreased (-0.003%) from the base build.
• Patch coverage: 4 uncovered changes across 3 files (31 of 35 lines covered, 88.57%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
spec/slack/web/api/mixins/users_spec.rb	4	2	50.0%
lib/slack/web/api/mixins/users.search.rb	1	0	0.0%
spec/slack/web/api/endpoints/custom/auth_spec.rb	1	0	0.0%
Total (14 files)	35	31	88.57%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	5997
Covered Lines:	5460
Line Coverage:	91.05%
Coverage Strength:	100.5 hits per line

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💛 - Coveralls