OAuth client: harden SEP-2352/SEP-2350 edge cases; fix conformance comment #2883

Job	Run time
checks / pre-commit	32s
checks / readme-snippets	14s
checks / test (3.11, locked, windows-latest)	1m 21s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.12, lowest-direct, windows-latest)	1m 30s
checks / test (3.10, lowest-direct, windows-latest)	1m 42s
checks / test (3.10, locked, windows-latest)	1m 54s
checks / test (3.10, locked, ubuntu-latest)	1m 5s
checks / test (3.10, lowest-direct, ubuntu-latest)	43s
checks / test (3.13, locked, ubuntu-latest)	57s
checks / test (3.14, lowest-direct, ubuntu-latest)	54s
checks / test (3.14, locked, ubuntu-latest)	2m 0s
checks / test (3.14, lowest-direct, windows-latest)	1m 28s
checks / test (3.13, locked, windows-latest)	1m 28s
checks / test (3.13, lowest-direct, windows-latest)	1m 34s
checks / test (3.13, lowest-direct, ubuntu-latest)	59s
checks / test (3.14, locked, windows-latest)	2m 42s
checks / test (3.11, locked, ubuntu-latest)	1m 0s
checks / test (3.12, locked, ubuntu-latest)	58s
checks / test (3.11, lowest-direct, windows-latest)	1m 26s
checks / test (3.12, lowest-direct, ubuntu-latest)	48s
checks / test (3.12, locked, windows-latest)	1m 31s
all-green	3s
	27m 51s

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