Add clear invisible captcha session

[dmrAnderson]

• Add clear_invisible_captcha_session to manually clear stale session data when a rendered form's action never runs (e.g.: OAuth login instead of the guarded form).
• Move session key strings into constants to stop typo drift between files.
• Ignore coverage output and exclude specs from the coverage report.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.17%. Comparing base (b0093de) to head (85def8d).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/invisible_captcha/controller_ext.rb	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   98.44%   97.17%   -1.27%     
==========================================
  Files          22        5      -17     
  Lines         449      177     -272     
==========================================
- Hits          442      172     -270     
+ Misses          7        5       -2     

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