OAuth client: harden SEP-2352/SEP-2350 edge cases; fix conformance comment #2904

Job	Run time
checks / readme-snippets	11s
checks / pre-commit	42s
checks / test (3.10, locked, ubuntu-latest)	52s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.11, locked, windows-latest)	1m 42s
checks / test (3.12, locked, windows-latest)	1m 46s
checks / test (3.14, locked, ubuntu-latest)	2m 5s
checks / test (3.13, locked, windows-latest)	1m 29s
checks / test (3.10, lowest-direct, windows-latest)	1m 42s
checks / test (3.12, locked, ubuntu-latest)	1m 6s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 7s
checks / test (3.12, lowest-direct, windows-latest)	1m 37s
checks / test (3.13, lowest-direct, windows-latest)	1m 29s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 3s
checks / test (3.14, locked, windows-latest)	2m 33s
checks / test (3.13, lowest-direct, ubuntu-latest)	1m 4s
checks / test (3.14, lowest-direct, ubuntu-latest)	56s
checks / test (3.10, locked, windows-latest)	1m 26s
checks / test (3.14, lowest-direct, windows-latest)	1m 28s
checks / test (3.13, locked, ubuntu-latest)	1m 1s
checks / test (3.11, locked, ubuntu-latest)	1m 4s
checks / test (3.11, lowest-direct, windows-latest)	1m 35s
all-green	5s
	29m 5s

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