index: make `ChangeIdIndex` methods fallible #6831

Job	Run time
check (rustfmt)	21s
check (codespell)	9s
check (protos)	2m 4s
check (cargo-deny, bans)	24s
check (clippy)	1m 32s
check (cargo-deny, licenses)	27s
check (cargo-deny, advisories)	28s
build (no git)	1m 30s
check (cargo-deny, sources)	31s
test (linux-x86_64-gnu)	6m 11s
test (linux-aarch64-gnu)	5m 33s
test (macos-aarch64)	5m 28s
nix flake (ubuntu-24.04-arm)	6m 59s
test (windows-x86_64)	14m 41s
test (macos-x86_64)	7m 46s
nix flake (ubuntu-24.04)	7m 40s
nix flake (macos-14)	11m 26s
test (windows-aarch64)	14m 36s
check (doctests)	3m 7s
check (mkdocs)	13s
check (latest mkdocs, optional)	7m 10s
check (zizmor)	34s
check (Cargo.lock dependency count)	5s
required checks (merge queue)	0s
	1h 38m 55s

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