Scope REST response metadata to the active session

[efegokdemir]

Description

Fixes #1365

REST resource response metadata is stored per thread, so a worker reused for another request can expose pagination or rate-limit values from the previous session. This change tags each response's metadata with the current session-lifecycle generation and makes the resource getters return nil when that generation changes. Values remain available throughout the same lifecycle.

How has this been tested?

• bundle _2.5.9_ exec rake test:library — 343 runs and 687 assertions passed.
• bundle _2.5.9_ exec rubocop lib/shopify_api/context.rb lib/shopify_api/rest/base.rb test/clients/base_rest_resource_test.rb — no offenses.
• The regression test failed on the unmodified implementation because next_page? stayed true after switching sessions; it passes with this change.
• Ruby syntax checks and git diff --check passed.
• Sorbet typechecking could not run: the locked static binary is x86_64 and this host is ARM.

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.