Add OIDC fallback for legacy OAuth discovery #2667

Job	Run time
checks / pre-commit	38s
checks / readme-snippets	15s
checks / test (3.10, lowest-direct, ubuntu-latest)	50s
checks / test (3.10, locked, windows-latest)	1m 17s
checks / test (3.11, lowest-direct, windows-latest)	1m 46s
checks / test (3.10, locked, ubuntu-latest)	49s
checks / test (3.11, locked, ubuntu-latest)	47s
checks / test (3.10, lowest-direct, windows-latest)	1m 14s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
checks / test (3.12, locked, ubuntu-latest)	51s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.12, locked, windows-latest)	1m 28s
checks / test (3.12, lowest-direct, windows-latest)	1m 21s
checks / test (3.11, locked, windows-latest)	1m 18s
checks / test (3.13, locked, ubuntu-latest)	49s
checks / test (3.13, locked, windows-latest)	1m 18s
checks / test (3.13, lowest-direct, windows-latest)	1m 27s
checks / test (3.14, lowest-direct, ubuntu-latest)	52s
checks / test (3.14, lowest-direct, windows-latest)	1m 9s
checks / test (3.13, lowest-direct, ubuntu-latest)	43s
checks / test (3.14, locked, ubuntu-latest)	1m 55s
checks / test (3.14, locked, windows-latest)	2m 27s
all-green	3s
	25m 10s

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