sorbet: enable experimental RSpec mode

Add `--enable-experimental-rspec` to the Sorbet config so that Sorbet
type-checks RSpec spec files. This is an isolated piece of the work in
#21690, separated so that the follow-up work of dialling up the `typed:`
level of individual specs can land independently.

Enabling the flag surfaced a pre-existing type error in
`formula-analytics/pycall-setup.rbi` where `InfluxDBClient3#initialize`
was incorrectly declared as a singleton method (`def self.initialize`)
rather than an instance method (`def initialize`). Fixed here since the
error is only visible with RSpec mode enabled.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dduugg

Library/Homebrew/formula-analytics/pycall-setup.rbi

@@ -1,7 +1,7 @@
 # typed: true
 
 class InfluxDBClient3
-  def self.initialize(*args); end
+  def initialize(*args); end
 
   def query(*args); end
 end

Library/Homebrew/sorbet/config

@@ -1,5 +1,6 @@
 --dir=.
 --enable-experimental-requires-ancestor
+--enable-experimental-rspec
 --ignore=/vendor/bundle
 --ignore=/vendor/gems
 --ignore=/vendor/ruby