Fix flakey cucumber specs

[cycomachead]

Pivotal Tracker Link

What this PR does:

This pull request fixes flaky Cucumber/Capybara specs that were intermittently failing in CI due to race conditions between test steps and asynchronous JavaScript initialization.

How should this PR be tested?

• Run the full Cucumber suite in CI (headless Chrome) and verify that previously flaky specs pass consistently across multiple runs.
• Key scenarios to watch: email modal close, TinyMCE content fill/read, languages dropdown select/remove, CSV file attachment, and hidden element assertions.

Root causes addressed:

• Slow JS initialization — Capybara.default_max_wait_time bumped from 2s to 10s to give DataTables, TinyMCE, Selectize, and Bootstrap modals time to initialize under CI load.
• Headless Chrome flags — Switched --headless to --headless=new; added --no-sandbox and --disable-dev-shm-usage for better CI resource handling.
• Email modal close — Replaced fragile sleep 0.2 + click sequence with a direct Bootstrap $('#emailModal').modal('hide') API call, eliminating the race between required-field validation and the close button click.
• TinyMCE race condition — Added a poll loop that waits for tinymce.editors.length > 0 before calling setContent, preventing undefined errors on async editor initialization.
• Ambiguous Selectize lookups — Scoped languages dropdown steps to #teacher_languages by ID rather than first(".selectize-input"), which was matching the school selectize widget on the same form.
• State leakage between scenarios — Wrapped Capybara.ignore_hidden_elements = false blocks in begin/ensure so a failing assertion can't leave the flag flipped and corrupt subsequent scenarios.

Are there any complications to deploying this?

No — test-only changes with no impact on application code.

Checklist:

• Has this been deployed to a staging environment or reviewed by a customer?
• Tag someone for code review (either a coach / team member)
• I have renamed the branch to match PivotTracker's suggested one (necessary for BlueJay) (e.g. michael/12345-add-new-feature Any branch name will do as long as the story ID is there. You can use git checkout -b [new-branch-name])

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.71%. Comparing base (daa7e96) to head (eb4769f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #426   +/-   ##
=======================================
  Coverage   74.71%   74.71%           
=======================================
  Files          29       29           
  Lines        1135     1135           
=======================================
  Hits          848      848           
  Misses        287      287           

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