gptel: add gptel-context-buffer-mode-map #950

Job	Run time
check (ubuntu-latest, 26.3, false)	45s
check (ubuntu-latest, 27.2, false)	50s
check (ubuntu-latest, 28.2, false)	1m 4s
check (ubuntu-latest, 29.3, false)	1m 11s
check (macos-latest, 28.2, false)	3m 11s
check (macos-latest, 29.3, false)	3m 26s
check (windows-latest, 26.3, false)	4m 44s
check (windows-latest, 27.2, false)	1m 44s
check (windows-latest, 28.2, false)	2m 28s
check (windows-latest, 29.3, false)	1m 31s
check (ubuntu-latest, snapshot, true)	44s
check (macos-latest, snapshot, true)	2m 57s
	24m 35s

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