gptel: add gptel-context-buffer-mode-map #953

Job	Run time
check (ubuntu-latest, 26.3, false)	42s
check (ubuntu-latest, 27.2, false)	59s
check (ubuntu-latest, 28.2, false)	1m 9s
check (ubuntu-latest, 29.3, false)	1m 12s
check (macos-latest, 28.2, false)	3m 27s
check (macos-latest, 29.3, false)	4m 0s
check (windows-latest, 26.3, false)	2m 56s
check (windows-latest, 27.2, false)	4m 2s
check (windows-latest, 28.2, false)	2m 36s
check (windows-latest, 29.3, false)	2m 54s
check (ubuntu-latest, snapshot, true)	48s
check (macos-latest, snapshot, true)	3m 16s
	28m 1s

Provide feedback