fix: align update operation delete api

[immanuwell]

this fixes a small matcher internal API mismatch.

docs/spec said DELETE /matcher/api/v1/internal/update_operation/{digest} and showed 200, but the handler actually does uuid.Parse on that path segment. so a client following the docs gets 400, which is kinda rough.

repro:

1. call DELETE /matcher/api/v1/internal/update_operation/sha256:0000000000000000000000000000000000000000000000000000000000000000
2. before this patch it returns 400 because the handler expects a UUID update operation id, not a digest
3. after this patch the contract is {id} and successful deletes return 204

tests: go test ./...