Add OIDC fallback for legacy OAuth discovery #3174

Job	Run time
checks / readme-snippets	14s
checks / docs	37s
checks / pre-commit	51s
checks / test (3.11, locked, ubuntu-latest)	1m 7s
checks / test (3.10, locked, ubuntu-latest)	1m 17s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 3s
checks / test (3.13, lowest-direct, windows-latest)	1m 40s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 19s
checks / test (3.11, locked, windows-latest)	1m 46s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 20s
checks / test (3.11, lowest-direct, windows-latest)	1m 40s
checks / test (3.13, locked, windows-latest)	1m 37s
checks / test (3.14, lowest-direct, windows-latest)	1m 44s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 11s
checks / test (3.12, lowest-direct, windows-latest)	1m 39s
checks / test (3.13, locked, ubuntu-latest)	1m 4s
checks / test (3.14, locked, windows-latest)	2m 20s
checks / test (3.12, locked, ubuntu-latest)	1m 23s
checks / test (3.10, locked, windows-latest)	1m 37s
checks / test (3.14, locked, ubuntu-latest)	2m 8s
checks / test (3.12, locked, windows-latest)	1m 41s
checks / test (3.10, lowest-direct, windows-latest)	1m 51s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 13s
all-green	2s
	32m 24s

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