v3.1.0

chore(release): 3.1.0 (DOT-3135) (#380)

Drop the .pre.1 suffix so the next release on legacy-3.x publishes the
stable 3.1.0: `~> 3.0` consumers, currently on 3.0.6, pick it up on a
normal bundle update. Content is the prerelease 3.1.0.pre.1 already on
rubygems.org (to_jwt from #372, faraday-retry dependency from #377),
with no code change.

Verified: `gem build telnyx.gemspec` produces telnyx-3.1.0.gem and
Gem::Version#prerelease? is false; bin/publish-gem refuses a mismatched
RELEASE_TAG before building. Full suite against the Prism mock: 593
tests, 0 failures, 20 errors that are identical on the untouched branch
head (spec drift on endpoints unrelated to this change).


Claude-Session: https://claude.ai/code/session_015xKTe1cqjTV5qCWLMam4VZ

Co-authored-by: Claude <noreply@anthropic.com>