chore(release): bump version to 0.12.0

[kxzk]

TL;DR

Prepare langfuse-rb 0.12.0 for the merged dependency security fixes and Faraday compatibility change.

Why

PR #115 prevents gem consumers from resolving vulnerable Faraday, JSON, and concurrent-ruby versions. It also requires Faraday 2.14.3 or newer and drops Faraday 1.x compatibility. The minor version bump makes that compatibility boundary explicit.

This pull request changes only CHANGELOG.md, Gemfile.lock, and lib/langfuse/version.rb. Tagging and gem building must wait until this pull request merges and its exact commit remains the head of main.

Linear: AAI-401

Checklist

• Has label
• Has linked issue
• Tests added for new behavior
• Docs updated (if user-facing)

Verification

• make check: 1,636 examples, 0 failures, 96.93% line coverage; RuboCop inspected 108 files with no offenses.
• bundle check: dependencies satisfied.
• Release metadata: version and lockfile report 0.12.0; changelog comparison links updated; Unreleased remains empty.
• git diff --check: passed.
• Langfuse CLI authenticated readback: HTTP 200 using credentials loaded directly from .env; the core-only query returned one observation.

The gem was not built or published. The release workflow reserves the build for the exact merged release commit, and RubyGems publishing remains a manual MFA-protected step.

---

Note

Low Risk
Release metadata only (version, lockfile, changelog). No runtime or security-logic changes in this PR.

Overview
Bumps langfuse-rb to 0.12.0 so the Faraday 2.14.3+ requirement and tighter Faraday/JSON/concurrent-ruby constraints from #115 ship as a minor release.

Updates Langfuse::VERSION, Gemfile.lock, and changelog comparison links. Documents Faraday 1.x drop and the vulnerable-dependency resolution fix under [0.12.0].

^{Reviewed by Cursor Bugbot for commit d9b606a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[linear-code]

AAI-401