✅ Add test for invalid mode via callable

[pboling]

@mrj This is how I added the test for invalid mode, in case you wondered.

[github-actions]

Package	Line Rate	Branch Rate	Health
oauth2	100%	100%	✔
Summary	100% (520 / 520)	100% (176 / 176)	✔

Minimum allowed line rate is 100%

[coveralls]

Pull Request Test Coverage Report for Build 17543842818

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

---

Totals
Change from base Build 17543399152:	0.0%
Covered Lines:	520
Relevant Lines:	520

---

💛 - Coveralls

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (9c20782) to head (4a13c92).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #672   +/-   ##
===========================
===========================

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