Fix NoMethodError / Make session_tracking check consistent

[knapo]

Description

#2245 (cc @st0012 ) introduced changes around checking if session_tracking is enabled or not.. However, it wasn't updated in all necessary places and make it inconsistent within the codebase. As result, when not enabled_in_current_env? we're getting a following error:

     NoMethodError:
       undefined method `add_session' for nil:NilClass
     # <REDACTED>/gems/sentry-ruby-5.17.0/lib/sentry/hub.rb:244:in `end_session'
     # <REDACTED>/gems/sentry-ruby-5.17.0/lib/sentry/hub.rb:253:in `with_session_tracking'
     # <REDACTED>/gems/sentry-ruby-5.17.0/lib/sentry-ruby.rb:403:in `with_session_tracking'

Affected version: sentry-ruby 5.17.0

[codecov]

Codecov Report

Merging #2269 (9a84215) into master (b05afd9) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2269   +/-   ##
=======================================
  Coverage   97.61%   97.61%           
=======================================
  Files         112      112           
  Lines        4155     4155           
=======================================
  Hits         4056     4056           
  Misses         99       99           

Components	Coverage Δ
sentry-ruby	98.30% <100.00%> (ø)
sentry-rails	95.05% <ø> (ø)
sentry-sidekiq	94.70% <ø> (ø)
sentry-resque	92.30% <ø> (ø)
sentry-delayed_job	95.60% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files	Coverage Δ
sentry-ruby/lib/sentry/hub.rb	99.38% <100.00%> (ø)

[st0012]

Ah sorry for the bug and thanks for the fix!

[ixti]

Figured out the reason for #2269 (comment)

It happens when we do in specs:

require "sentry/test_helper"

RSpec.configure do |config|
  config.include Sentry::TestHelper, sentry_test: true
  config.before(sentry_test: true) { setup_sentry_test }
  config.after(sentry_test: true) { teardown_sentry_test }
end

[ixti]

Okay. Found how to reproduce the error:

Sentry.init do |config|
  # ...

  config.enabled_environments = %w[staging production]

  # ...
end

Then using in specs:

require "sentry/test_helper"

RSpec.configure do |config|
  config.include Sentry::TestHelper
end

RSpec.describe "..." do
  before { setup_sentry_test }
  after { teardown_sentry_test }

  # ...
end

setup_sentry_test injects "test" into environments