Gracefully handle an exchange delegate evicting the session on timeout.

[bzbarsky-apple]

Otherwise the session eviction (which closes our exchange) would destroy it, and then we would unwind into ExchangeContext::NotifyResponseTimeout and try to use the exchange's members.

[github-actions]

PR #25908: Size comparison from 522b78b to a592938

Increases (1 build for cc32xx)

platform	target	config	section	522b78b	a592938	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	(read only)	645769	645825	56	0.0
			.debug_info	20310042	20310330	288	0.0
			.debug_line	2681003	2681125	122	0.0
			.debug_loc	2827360	2827568	208	0.0
			.debug_ranges	286352	286448	96	0.0
			.text	537696	537752	56	0.0

Full report (1 build for cc32xx)

platform	target	config	section	522b78b	a592938	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645769	645825	56	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933102	933102	0	0.0
			.debug_aranges	87704	87704	0	0.0
			.debug_frame	301596	301596	0	0.0
			.debug_info	20310042	20310330	288	0.0
			.debug_line	2681003	2681125	122	0.0
			.debug_loc	2827360	2827568	208	0.0
			.debug_ranges	286352	286448	96	0.0
			.debug_str	3041144	3041144	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105953	105953	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	380543	380543	0	0.0
			.symtab	257472	257472	0	0.0
			.text	537696	537752	56	0.0

[github-actions]

PR #25908: Size comparison from 411b698 to 4daaee9

Increases (1 build for cc32xx)

platform	target	config	section	411b698	4daaee9	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	(read only)	645769	645825	56	0.0
			.debug_info	20310042	20310332	290	0.0
			.debug_line	2681003	2681125	122	0.0
			.debug_loc	2827360	2827568	208	0.0
			.debug_ranges	286352	286448	96	0.0
			.text	537696	537752	56	0.0

Full report (1 build for cc32xx)

platform	target	config	section	411b698	4daaee9	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	645769	645825	56	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933102	933102	0	0.0
			.debug_aranges	87704	87704	0	0.0
			.debug_frame	301596	301596	0	0.0
			.debug_info	20310042	20310332	290	0.0
			.debug_line	2681003	2681125	122	0.0
			.debug_loc	2827360	2827568	208	0.0
			.debug_ranges	286352	286448	96	0.0
			.debug_str	3041144	3041144	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105953	105953	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	380543	380543	0	0.0
			.symtab	257472	257472	0	0.0
			.text	537696	537752	56	0.0