build(deps): bump rack-session to 2.1.2 (GHSA-33qg-7wpp-89cq) (#67)

Bumps the transitive rack-session dependency from 2.1.0 to 2.1.2 to
resolve a critical advisory: Rack::Session::Cookie's decrypt-failure
fallback enables secretless session forgery and Marshal deserialization
(vulnerable range >= 2.0.0, < 2.1.2).

Conservative lockfile-only change; no direct Gemfile dependency touched.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: technicalpickles

Gemfile.lock

@@ -202,7 +202,7 @@ GEM
     public_suffix (6.0.1)
     racc (1.8.1)
     rack (3.1.12)
-    rack-session (2.1.0)
+    rack-session (2.1.2)
       base64 (>= 0.1.0)
       rack (>= 3.0.0)
     rack-test (2.2.0)